Abstract: To correctly simulate laser propagation in turbulence with phase screen method, based on the turbulent atmosphere-s property, the sampling principle and the grid intervals- relation between two adjacent screens determined by FFT, a formula to calculate the optimal value for grid number based on step distance and grid intervals is derived, and laser propagation in free space and turbulent atmosphere is simulated. It is shown that the simulation result for laser propagation in free space coincides with the theory result when grid number-s value is 848 calculated by the given formula. Specifying grid number a smaller or lager value such as 836 or 860, it is equivalent to that light ray is converged or diverged by a convex lens arrays or a concave lens arrays when passing through even screens, which weaken or exaggerate turbulence-s effect on propagation beam and generate erroneous simulation result.