Abstract: The rapid cooling down stress deformation is one of the key factors for the reliability of FPA. Combined with chemical corrosion, a mechanical grinding method was studied in this paper to reduce the thickness of FPA chip, and therefore improving its flexibility. The amount of bad pixel and crack owing to pixel damage generated by stress transfer was reduced obviously in the thinned chips. The cold head expansion matched to decrease the cool down stress of the chip was also optimized, without increasing the structural components of the Dewar cold head. After verified by reliability testing, the long-term reliability and stability of the 320256 IRFPA detector under a rapid cooling startup was improved obviously by these optimizations.