Abstract: In order to analyze the surface deformation of the frame scanning mirror of a certain thermal imager in operation mode, the cause of the 2 KHz resonance was investigated for improving the design basis. The scanning angular velocity and angular acceleration of the frame scanning mirror were calculated based on the moving curves of the frame scanning mirror，the acceleration, deceleration, the surface deformation as well as the first ten orders of the natural modal of the scanning mirror in the process of scanning were calculated with FEA software (IDEAS), and the cause of 2 KHz resonance of the scanning mirror was analyzed in the experiment. Based on the experimental results, the surface deformation and the first 10 orders of the natural modal were calculated, and the structure of the scanning mirror was improved. It was proved that the resonance phenomenon was eliminated. At last, the important details are given in designing the scanning mirror, care shall be given to the deformation of the facet and efforts shall be made to avoid the jitter from driving torquer, which will interfere the natural modal.