An optical readout system based on Hartmann-Shack sensor is introduced. This system uses Shacke-Hartmann wavefront sensor (SHWS) to detect the wavefront gradient variance of focal-plane array (FPA) cell before and after being heated and then reconstructs the IR image of object being detected. After vividly discussing its imaging capability theoretically, we obtain the infrared image of a high-temperature object with cell size 60 μm×60 μm and array size 34 pixel×38 pixel in experiment. Its noise equivalent temperature difference (NETD) is about 3.8 K. If system parameters are optimized, the measurement accuracy of SHWS for arriving angles can be improved and thus smaller NETD can be attained.