Cube corner reflector is widely used as the cooperative target in laser tracking, location and other systems, its far field diffraction intensity distribution is of great significance to the design of cooperative target as well as to the theoretical study on the performance of laser radar system. An analytic algorithm for calculating the diffraction intensity is presented in this paper, in which, the effective reflective area of corner reflector is approximately taken as an ellipse, and the reflected beam is decompose of a series of complex Gaussian functions. Thus the far-field diffraction field is expressed as the superposition of a series of elliptic Gaussian beam. Numerical results show that the relative error caused by this algorithm is less than 5% in most cases, therefore, this algorithm is a feasible.