The intensity fluctuation in space laser communication is analyzed quantitatively by using numerical simulation method, the relation of the intensity correlative scale with the atmospheric coherence length is obtained. And the referenced factors of ascertaining optimum receiving aperture are discussed. The optimum receiving aperture is defined base on the ITU-T, and the relation of optimum receiving aperture with atmospheric coherent length is educed, the result shows that the optimum receiving aperture is greatly increasing as the atmospheric coherence distance decreasing. When focused laser beam is used in space laser communication, at the strong atmospheric turbulent effect, for example r0 about 4 cm, only the receiving aperture approaches 5 times diameter of Airy spot, the lowest ITU-T demand can be satisfied.