In the design of high‐power microwave (HPM) waveguide array of longitudinal shunt slots, the mutual coupling of slots in different waveguide can badly decrease the capability of wide‐angle scanning of array. An L‐band HPM waveguide array of longitudinal shunt slots is designed. The choke groove was designed to restrain the mutual coupling of slots of the HPM array antenna. The result of simulation verified that the scanning angle with the gain of antenna decreased of 3 dB of array without choke groove structure is only 25°, and that is 35.0° with choke groove structure. The wideband of active reflection of array can be improved by the choke groove structure. The result of experiment also verified that the slot array antenna provided 30° beam steering with a gain reduction of 1.8 dBi and the power‐handling capacity of the array with the choke groove is greater than 1.5 GW with width of pulse of 42 ns. © 2023 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.