Abstract: In this paper, anisoplanatism in adaptive optics and capability of phase derivative adaptive optics are analysed in time domain. The general formulas of residual phase structure function and Strehl ratio are obtained, which not only be used to discuss the anisoplanatism, but also can be used to analyse the other factors that degrade adaptive optics system performance, such as the mean wind velocity and the finite servo bandwidth of adaptive optics system. The results show that the capable scale of phase derivative adaptive optics are very narrow.