In the optical readout infrared thermal imaging system, the initial reflector deformation of the focal plane array (FPA) decreases the optical detection sensitivity for the system greatly. According to the design and fabrication of the FPA, two optimized design schemes for reducing the initial deformation of the FPA reflector are put forward: thinning the Au layer on the reflector and fabricating reflector with stiffeners. Based on the theoretical analysis, a FPA with a unit size of 200 μm is fabricated and the Au layer of its reflector is thinned. Its reflector curvature radius and the optical detection sensitivity for the system are respectively increased to 4.71 times and 5.2 times of the FPA without thinning process. And a FPA with a unit size of 60 μm is fabricated, its reflector is with stiffeners. Its reflector curvature radius and the optical detection sensitivity for the system are respectively increased to 4.29 times and 1.18 times of that of the FPA without stiffeners. The experiments verified the result of theoretical analysis.