In order to solve the problem of long distance energy transmission, divergence angle of laser transmitted by space-borne laser communication terminal must be usually very small, typically on the order of tens of microradians. But the ground testing must properly amplify the divergence angle, to ensure that the communication terminal can always in the beacon beam and communication beam range with a certain tracking accuracy. This paper uses the communication light which sensitive the intensity distribution as an example, building the theory molding and simulating for different distance and position, analyzing the influence, which provides scientific basis for space laser communication ground demonstration. Meanwhile, defocus and magnification of two kinds of divergence angle amplification method was compared. In the ground testing of a space laser communication, we give the optimal selection scheme. The result provides scientific basis and accumulated experience for the space laser communication ground test.