Abstract: Base on imaging characteristics of the adaptive optical telescopes, the number of optimum subapertures is defined and analyzed when the brightness of the image spot center is maximal and the residual wavefront error is minimal. The wavefront correction of the deformable mirror with the triangular and rectangular actuator distribution was simulated by ZEMAX. The quantity of the optimum subapertures for the 600mm adaptive optical telescope was derived based on the analysis of the two cases that the adaptive telescope has or has not a fast tilting mirror.