To address the wideband and beam scanning requirements of high-power microwave (HPM) systems, this paper proposes an X-band varactor-based high-power wideband beam-scanning reflectarray antenna. The antenna employs a linearly polarized horn feed and a sandwich-structured embedded patch element, where the nested dual-resonance structure integrated with varactors simultaneously extends the phase tuning range (360°) and operational bandwidth. By eliminating abrupt structural discontinuities and adopting a sandwich dielectric configuration, the design effectively suppresses triple-junction formation, achieving a power capacity of 5 MW in 0.1 MPa SF₆ environment. Varactor capacitance adjustment enables a 12% relative tuning bandwidth within 8.55-9.65 GHz. Simulations of an 11×11 rectangular grid reflectarray demonstrate a maximum gain of 25.12 dBi with 54.39% aperture efficiency for a 242 mm aperture and full-band 0°-20° beam scanning capability. Compared with existing technologies, this design exhibits superior tuning bandwidth (12%) and power capacity (5 MW), providing an effective solution for wideband beam control in HPM systems.