Aiming at the application requirements of array antenna with high-power capacity, high efficiency and low profile characteristics, a high-power capacity and high efficiency open waveguide array antenna is proposed and designed. The antenna consists of a compact 16-way waveguide power distribution network, 4×4 rectangular open waveguide unit cells and ceramic sealing radome. By designing the size of the open waveguide and loading E-plane metal bar on the surface of the open waveguide, the electric field distribution on the radiation aperture surface is more uniform, and the radiation gain of the unit cell is improved. The step matching structure is used to realize the size transformation from the output port of the waveguide power distribution network to the interface of the open waveguide unit cell, and the impedance bandwidth of the system is improved. The ceramic radome loaded on the array keeps the interior of the antenna in a vacuum state and improves the power capacity of the antenna. According to the application requirements of X-band high-power array antenna, a 16-element open waveguide array with a center frequency of 9.5 GHz is optimized and designed, the simulation results show that the aperture efficiency is greater than 90% and the reflection coefficient is less than -13.9 dB in the range of 9.25-9.65 GHz. The antenna is processed and tested, the measured antenna reflection curve and radiation pattern at the center frequency are in good agreement with the simulation results, the antenna gain at the center frequency is 21.7 dBi. The overall profile height of the antenna is twice the wavelengths at the central frequency, and the power capacity in vacuum obtained by simulation is 40 MW, the antenna has the characteristics of high power capacity, high efficiency and low profile.