Abstract: To decrease the influence of mutual coupling between the slots of waveguide narrow wall slot antenna and implement the design of low sidelobe level, a design method of the wave distribution of inclined angles is proposed. The Taylor distribution is used as the current distribution of the antenna, the initial inclined angles are obtained by the HFSS, based on which the wave distribution of inclined angles is proposed, and the inclined angles are optimized by genetic algorithm. The co-simulation of Matlab and HFSS is adopted, which omits a large number of operations of creating models and setting up parameters, thus the efficiency of designing is clearly improved. With the method, a 1.2 meters long waveguide narrow wall slot antenna operating at X band is designed. The simulated result indicates that the sidelobe level is 2.17 dB lower than that obtained by the traditional design method. Finally, the antenna is manufactured and tested, and the tested gain and sidelobe level of the antenna are 20.9 dBi and-28.7 dB respectively, and the effectiveness of which is verified.