A high-intensity continuous-wave (CW) radio frequency quadrupole (RFQ) accelerator is designed for boron neutron capture therapy. The transmission efficiency of a 20-mA proton beam accelerated from 30 keV to 2.5 MeV can reach 98.7％ at an operating frequency of 200 MHz. The beam dynamics have a good tolerance to errors. By comparing the high-frequency parameters of quadri-lateral and octagonal RFQ cross sections, the quadrilateral structure of the four-vane cavity is selected owing to its multiple advantages, such as a smaller cross section at the same frequency and easy processing. In addition, tuners and undercuts are designed to tune the frequency of the cavity and achieve a flat electric field distribution along the cavity. In this paper, the beam dynamic simulation and electromagnetic design are presented in detail.