The development of reflectarray antenna is limited by bandwidth and power capacity. In this paper, a novel reflectarray element based on multi-resonant technique is proposed firstly. Compared with the traditional element, the proposed element structure has the characteristics of high-power capacity, low profile, and good linearity of phase shift curve. Secondly, a reflectarray antenna with 20 × 20 elements is designed in Ka band by using the proposed element and optimizing the array characteristics. Finally, the electromagnetic simulation software is used to simulate the antenna, The results show that at the center frequency of 35 GHz, the peak gain of the antenna is 27.58 dB, the aperture efficiency is 52.33%, the sidelobe is less than -16.08 dB, and the antenna gain drop is less than 3 dB in the frequency range of 30.41-39.64 GHz (the relative bandwidth is 26.37%). Moreover, the power capacity of the designed reflectarray reaches 13.99 MW and the power density is 218.54 W/mm².