In order to improve the spatial resolution of the optical microscanning thermal microscopic system, the zero of microscanning should be determined to complete the oversample reconstruction. Based on the geometric principle and digital imaging processing, a new zreo calibration was presented. The definition, the working principle and the method were analyzed, and the microscanning zero calibration of the real system was done. Different reconstruction methods were applied to reconstruct the thermal image and the real thermal microscopic image, moreover the evaluation parameters were given. Results of simulation and real thermal imaing peocessing show the availability of the zero calibration, thus the high resolution optical microscanning thermal microscopic system has been achieved. The system can be applied into many systems which need microscopic thermal anslysis with high spatial resolution.