The correction of the aberrations in light path of adaptive optical system is of great importance. However, the diffraction effect of the laser beam propagation in the adaptive optical system limits the correction effect of the aberration. By means of stochastic parallel gradient descent, correction effect under different Fresnel numbers is calculated, and an improved method is proposed. The simulated and experimental result indicates that the compensative effect decreases with the decrease of Fresnel numbers, and the correction effect of the improved solution is better than the traditional solution, and little influence of the diffraction effect is on the correction effect of the aberration with the former.