Abstract: This paper investigates the application of waveguide slot array antennas in high-power microwave technology and proposes a novel design method, with particular emphasis on the slot coupling, sidelobe levels, and matching between the antenna and the feed. The new method leverages modern computing technology to rapidly compute the slot conductance function considering slot coupling effects, thereby enabling efficient design of waveguide slot array antennas. This method avoids complex calculations or external structures, ensuring system compactness and demonstrating high effectiveness in designing waveguide slot planar arrays. Simulation results indicate that antennas designed using the new method exhibit excellent matching performance. At the center frequency f = 2.458 GHz, the reflection coefficient for each port of antenna designed using the new method ranges from −37.2 dB to −27.7 dB. Compared with the range from −11 dB to −8.7 dB of antennas designed using the Stevenson formula for the same target parameters, the reflection coefficient of antennas designed with the new method is reduced by at least 19 dB. Moreover, the antennas designed with this new method achieve a low sidelobe level of −30.2 dB and a high power capacity of 332.6 MW.