Abstract: The paper presented a method for fast computation of the radiation pattern of the two-dimensional planar phased array with rectangular grid, which is based on Fast Fourier Transform （FFT）. The strict mathematical derivation is given to show the intrinsic relationship between planar array pattern and two-dimensional Fb~F. In virtue of the conception of visible space, the applied technique of FFT algorithm on the fast computation of planar array pattern is demonstrated. Meanwhile, examples are given to validate its precision and speed. At last, some details are described, such as the pattern transacting at a specific cut plane, analysis on accuracy of scanning angles, computation of directional coefficient.