To meet the research demand of high-power microwave system for wide frequency ratio dual-band antenna, this paper proposes a high-power circularly polarized reflectarray antenna that can work in C/X dual-band. The antenna unit adopts the form of substrate buried patch, and the patch part consists of an elliptical ring patch nested with an elliptical patch in the inner to realize the radiation of low frequency (C-band) and high frequency (X-band) respectively. This nested cell form enables the antenna to achieve a wide frequency ratio, at the same time, because the cell has no abrupt structure and it is buried in the substrate to avoid the emergence of triple junction, it has a high power capacity. The two patches of high and low frequency bands rotate around the axis to adjust the reflection phase, which can meet the reflection phase adjustment of 360° on the basis of small reflection loss. Based on the above dual-band radiation unit, design and optimization of a 20×20 rectangular grid reflectarray antenna with an aperture size of 400 mm×400 mm are made. While the antenna works at 4.3 GHz, the gain is 22.2 dBi, the aperture efficiency is 40.2%, and the power capacity is 10.4 MW in air condition. At 10.4 GHz, the antenna has a gain of 29.9 dBi, an aperture efficiency of 40.5%, and a power capacity of 12.2 MW in air condition. the frequency ratio between high and low working frequencies of the antenna reaches 2.4, and it has the characteristics of high efficiency and high power capacity.