By using the direct wavefront gradient control method, numerical simulation of all parts of an adaptive optics (AO) system is carried out. We present “phase cutting” of the calculated phase to solve the problem that variance of the calculated residual wavefront is not corresponding to the correction effect of AO system. The computational results in imaging of lenses by means of the fast Fourier transform (FFT) method agree excellently with those by means of integration method. It is found for the first time that for a constant delay time of an AO system, the compensation effect of the AO system is better than that of complete phase compensation when the velocity of lateral wind exceeds a threshold. It is shown that a better compensation capability of an AO system does not mean a better correction effect.