Firstly time-variable phase screen is introduced into the split-step Fourier method，and dynamic speckle field reflected by a fully diffused surface in vacuum is simulated.The spatial and temporal correlation functions are calculated，and the effects of the beam radius and the velocity of the target on speckle characteristics are analyzed.Secondly based on the theory of frozen field，the dynamic speckle field under atmospheric turbulence is simulated by the split-step Fourier method，and the effects of wind velocity on speckle characteristics are analyzed.The results show that the speckle size is determined by the size of the laser spot on the target plane.The speckle size decreases with increase of laser spot size.The correlation time is affected by the size of the laser spot and the phase of the filed on the target plane.The correlation time gets shorter as the phase of the field changes faster.