This paper proposes a Ku-band phased array antenna element for high-power microwave application. On the basis of the traditional horn antenna and coaxial to waveguide converter, the cross-shaped transition cavity structure and the square metal matching block structure are introduced, and the impedance matching is realized by using the impedance transformation principle in the microwave network. At 12.5GHz-14.5GHz, the VSWR is less than 2. The power capacity of the antenna unit is 32.3kW and has been increased by 300%. This design makes up for the problem of insufficient transmit power of Ku-band high-power phased array, and has certain high-power application value.