Abstract: The 1280×1024 mercury cadmium telluride (MCT) midwave infrared (MWIR) focal plane array (FPA) detector module employs a layered structure comprising an MCT detector chip, silicon readout integrated circuit (ROIC), lead wire substrate, and Dewar cold finger. When the detector is cooled to below room temperature (90 K), thermal mismatch stress develops at the material interfaces because of differences in the thermomechanical parameters, potentially causing performance degradation or structural failure. To address the low-temperature reliability issues in the 1280×1024 HgCdTe MWIR focal plane array detector assembly, a finite element model of the assembly structure was developed using the multiphysics coupling analysis module in ANSYS WORKBENCH. The model was employed to analyze the variation in thermal stress within the detector during the Dewar packaging process. By appropriately selecting the materials for the substrate and cold head and optimizing the structural dimensions, a packaging configuration suitable for engineering applications was achieved. Under the conditions of an Invar cold head and an Al₂O₃ ceramic substrate with a thickness exceeding 2 mm, the 1280×1024 HgCdTe MWIR focal plane detector assembly demonstrated a thermal cycling of over 300 cycles.