Ultra thin mirror has a large diameter-thickness rate, so it will be more deformable because of the change of work surroundings. In this paper, some analysis and computing simulation for active ultra thin mirror surface by using Finite Element Method (FEM) were introduced. An ultra thin mirror with a large ratio was simulated by software "patran". The aberrations gotten with the Zernike polynomials several low-orders were used to simulate the surface error after ultra thin mirror fabrication and the deformation because of change of work surroundings. After getting the response function of single actuator, the optimum scheme of actuator array for an ultra thin active mirror with maximum corrective capability was achieved. At last, the optimal thickness of the mirror was determined.