Abstract: The coupler, which belongs to the field of proton linear accelerator technology, was designed for a four-vane radio frequency quadrupole (RFQ) accelerator as a high power coupling device. It adopts a new bowl-shaped ceramic window structure specially, and its reasonable structural design provides a good solution to transmission impedance matching. The coupler consists of inner conductor, outer conductor, ceramic window, coupling loop and complicated water cooling channels. And the nitride titanium coating on the surface of ceramic window can prevent temperature rising from multipacting phenomenon. The radio frequency (RF) power achieved rated value of 120 kW through two identical compact couplers on the RFQ cavity in July 2014, and the measured results meet the requirements of original dynamics design of 2.1 MeV output energy and 10 mA beam current. The design of the coupler structure provides an abundant technical reference for the RF design of couplers with the similar power and frequency scale in civil related field.