A fast beam optimization algorithm based on sampling synthesis initial value is proposed for radar phased array antenna. In this method, the main beam or shaped region of the desired pattern is sampled, and the sampling points are considered to be multiple groups of scanning patterns. Therefore, the basic weights of the multi-group pattern are synthesized as the intial value of the optimization method. Then the pattern is calculated by FFT, and compared with the desired pattern, the expected value is directly replaced by the expected value for the region that does not meet the expectation. Repeat the pattern calculation and the inversion operation until the pattern meets the design requirements. The method has the advantages of simple operation, fast convergence, practical and efficient.