According to engineering needs, this paper proposes a new type of high-power microwave active phased array antenna unit. The power capacity and standing wave characteristics are improved by modifying the structure of the coaxial waveguide converter. A square metal block was introduced between the coaxial probe and the stepped waveguide to achieve impedance matching, a transition cavity and matching slot structure were added at the end of the waveguide, and a single ridge structure was added to the horn section to change the waveguide impedance. The single ridge extends out of the aperture surface to reduce the reflection coefficient and further improve the power capacity. In X-band, the VSWR is less than 2, the bandwidth is improved to 1.25GHz, and the power capacity is 33.24kW, which is 230% higher than that of the horn antenna under the conventional coaxial waveguide converter. The 7×7 phased array can achieve MW-level transmission power and has the ability to transmit high-power microwaves. The array can achieve ±30° conical scanning to meet the needs of high-power wide-angle scanning.