Abstract: The equivalent admittance is a key design parameter of the waveguide slot antenna. The traditional waveguide slot array antennas have higher side lobe and longer design time. An improved design of single ridge waveguide slot arrays is proposed in this paper on the basis of the traditional waveguide slot antenna design. The technique by adding metal diaphragms under slot units is presented to fit the influence of the equivalent admittance on the equivalent circuit of antenna. We can change the height of the metal diaphragm to optimize the admittance of each slot, so that the aperture field distribution of this novel design is closer to the theoretical design value. The admittance is extracted by using Newton iterative method to shorten design time. Simulated results show that the side lobe level of the antenna can be suppressed effectively with this method, the side lobe level can be below -30 dB, and a waveguide slot antenna array can be designed easily and economically in order to meet the requirements of the main indicators.