Accurately simulating the effect of atmospheric turbulence on the laser beams will improve the imaging ability of practical synthetic aperture ladar. A formalism to model the phase screen(PS) distorted by atmospheric turbulence based on fractal theory is provided. The random midpoint motion(RMM) algorithm is introduced and used to simulate the phase screens corresponding to Kolmogorov and non-Kolmogorov turbulence respectively. Simulation accuracy evaluation is carried out by comparing the phase structure function and the theoretical results. Power spectrum method and structure function method are also used. Results show that RMM algorithm can give a better approximation to the theoretical value.