In this paper, utilizing the geometrical phase shifting method of Variable Rotation Technique (VRT), we propose a beam scanning high-power microwave reflectarray antenna based on the concept of transmission phase difference. Electromagnetic simulation results show that the designed trident-shaped reflectarray antenna element operates at 9.5-10.5 GHz, has a linear phase shift capability within 360° at 0-40° incidence angle, and has a power handling capacity of 1.11 GW under vacuum conditions. A circular-shaped aperture reflectarray antenna with a radius of 200 mm is designed using the proposed element, and verified by full-wave simulation. Through the reconfiguration of the aperture phase distribution, the designed reflectarray antenna can realize the beam scanning in the range of ±40°. At 10 GHz, the maximum gain loss during beam scanning is less than 1.63 dB, the maximum gain reaches 31.1 dBi, the corresponding aperture efficiency is 73.42%, while the minimum aperture efficiency is more than 50%. The sidelobe level and axial ratio are always lower than -18.7 dB and 1.6 dB, respectively.