diff --git a/angelslim/utils/config_parser.py b/angelslim/utils/config_parser.py
index fb2f7fed..b0244b43 100644
--- a/angelslim/utils/config_parser.py
+++ b/angelslim/utils/config_parser.py
@@ -755,6 +755,25 @@ def parse_json_compression_config_section(compress_config: dict) -> CompressionC
     return CompressionConfig(name=comp_names, quantization=quantization, cache=cache)
 
 
+def _require_json_section(config_data: dict, section_name: str) -> Any:
+    """
+    Fetch a required top-level section from a JSON configuration.
+
+    Raises a descriptive ValueError (instead of a bare KeyError) when the
+    section is missing, mirroring the validation style of the YAML parser.
+
+    Args:
+        config_data: The full parsed JSON configuration dictionary.
+        section_name: Name of the required top-level section.
+
+    Returns:
+        The section value associated with ``section_name``.
+    """
+    if section_name not in config_data:
+        raise ValueError(f"Missing required '{section_name}' section in JSON configuration.")
+    return config_data[section_name]
+
+
 def parse_json_full_config(json_file_path: str) -> FullConfig:
     """
     Parses a JSON configuration file into a FullConfig instance
@@ -769,10 +788,12 @@ def parse_json_full_config(json_file_path: str) -> FullConfig:
         config_data = json.load(f)
 
     # Parse model configuration section
-    model_config = ModelConfig(**config_data["model_config"])
+    model_config = ModelConfig(**_require_json_section(config_data, "model_config"))
 
     # Parse compression configuration section
-    comp_config = parse_json_compression_config_section(config_data["compression_config"])
+    comp_config = parse_json_compression_config_section(
+        _require_json_section(config_data, "compression_config")
+    )
 
     # Parse other configuration sections with default fallbacks
     dataset_config, global_config, infer_config = (
@@ -805,26 +826,9 @@ def parse_json_full_config(json_file_path: str) -> FullConfig:
         if spin_data is not None:
             transform_config.spin_config = SpinConfig(**spin_data)
 
-    # Parse calibration configuration section (nested under compression)
-    comp_data = config_data.get("compression_config", {})
-    calibrate_data = comp_data.get("calibrate", None)
-    if not calibrate_data and config_data.get("calibrate_config"):
-        # Backward compatibility: support top-level calibrate_config
-        calibrate_data = config_data["calibrate_config"]
-    if calibrate_data:
-        comp_config.calibrate = CalibrateConfig(**calibrate_data)
-
-    # Parse transform configuration section
-    transform_config = None
-    transform_data = config_data.get("transform_config", {})
-    if transform_data:
-        spin_data = transform_data.pop("spin_config", None)
-        transform_config = TransformConfig(**transform_data)
-        if spin_data is not None:
-            transform_config.spin_config = SpinConfig(**spin_data)
-
     return FullConfig(
         model_config=model_config,
+        compression_config=comp_config,
         dataset_config=dataset_config,
         global_config=global_config,
         infer_config=infer_config,
diff --git a/tests/test_config_parser.py b/tests/test_config_parser.py
new file mode 100644
index 00000000..9fdfb594
--- /dev/null
+++ b/tests/test_config_parser.py
@@ -0,0 +1,65 @@
+# Copyright 2025 Tencent Inc. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Unit tests for JSON configuration parsing in ``angelslim.utils.config_parser``.
+
+These tests are CPU-only and require neither a GPU nor model weights: they
+exercise the pure configuration-parsing logic that ``Engine.prepare_model``
+relies on when loading a previously compressed model from
+``angelslim_config.json``.
+"""
+
+import json
+
+import pytest
+
+from angelslim.utils.config_parser import parse_json_full_config
+
+
+def _write_json(tmp_path, payload):
+    config_path = tmp_path / "angelslim_config.json"
+    config_path.write_text(json.dumps(payload))
+    return str(config_path)
+
+
+def test_json_roundtrip_preserves_compression_config(tmp_path):
+    """The compression section must survive a JSON load round-trip.
+
+    ``Engine.prepare_model`` forwards ``slim_config.compression_config`` to
+    ``from_pretrained``; if it is dropped during parsing the compressed model is
+    reloaded without any compression metadata.
+    """
+    payload = {
+        "model_config": {"name": "Qwen", "model_path": "Base Model Path"},
+        "compression_config": {
+            "name": "PTQ",
+            "quantization": {"name": "fp8_dynamic", "bits": 8},
+        },
+        "global_config": {"save_path": "Save Model Path"},
+    }
+
+    full_config = parse_json_full_config(_write_json(tmp_path, payload))
+
+    assert full_config.compression_config is not None
+    assert full_config.compression_config.name == ["PTQ"]
+    assert full_config.compression_config.quantization is not None
+    assert full_config.compression_config.quantization.name == "fp8_dynamic"
+
+
+def test_json_missing_required_section_raises_value_error(tmp_path):
+    """A missing required section should raise a descriptive ValueError."""
+    payload = {"model_config": {"name": "Qwen", "model_path": "Base Model Path"}}
+
+    with pytest.raises(ValueError, match="compression_config"):
+        parse_json_full_config(_write_json(tmp_path, payload))