Abstract: Aiming at the problem of large sparse array antenna synthesis without the knowledge of the expected pattern, a method based on off-grid with particle swarm optimization(PSO) to synthesize the position of sparse array antenna to maximize the gain of array is proposed in this paper. Based on the algorithm of PSO, this method carries on the Gradient searching again in the grid cell and finds the position of array unit rapidly when the gain of array is maximum without the knowledge of the expected pattern. The effect of synthesizing the sparse array by this method is fine and the improvement of gain is obvious without grating lob while scanning. The result of HFSS full wave simulation agrees well with the synthetic result, and thus, the validity of this method is proved.