Traditional methods of array antennas beamforming are doing some optimization to array factors. However, these methods ignore the effect of array elements, which result in the designed antenna patterns having some difference from the required patterns. In this paper, a method of combining differential evolution algorithms (DE) and high frequency structure simulator (HFSS) is used to make a low sidelobe beamforming for a microstrip slot antenna array. The method considers the effect of each array element. The difference which proves the method is feasible and valid between the designed antenna pattern and the simulation of HFSS is less than 0.3 dB.