Aiming at the beam optimization problem of waveguide optical phased array, we propose a sidelobe optimization method with mixed weighting of amplitude and phase based on genetic algorithm. Taking one-dimensional linear array as an example, we analyze the implementation method of mixed weighting. On the basis of the analysis, the MATLAB simulation in computer is carried out for the proposed genetic optimization algorithm. The simulation analysis shows that the optimized antenna pattern compresses the grating-lobe to the sidelobe. Meanwhile, the peak sidelobe level is greatly ameliorated. We perform a comparative analysis of whether the main lobe is included in the optimized search process, the optimized simulation results show that both of them can achieve the purpose of compressing the sidelobe. This algorithm has good convergent performance and can improve the efficiency and quality of the design by optimizing the design of the direction diagram. It has very good practical engineering applications in the direction diagram synthesis.