Abstract: The input power and duty factor of boron neutron capture therapy test equipment_A (BNCT_A) RFQ are respectively 420 kW and 50%. In order to meet the power requirements of the RFQ cavity, a high power RF coupler was designed. This coupler includes WR2300 waveguide-coaxial transformer, ceramic window, and power coupling equipment. The ceramic window employs Choking structure type and power coupling equipment is a magnetic loop. The segmented design method was used in the design of the power coupler. The waveguide-coaxial transformer and ceramic window were respectively optimized by using Microwave Studio (MWS). The standing wave ratio (SWR) of the ceramic window is 1.000 9@352.2 MHz, and the bandwidth is more than ±58 MHz@SWR≤1.1 MHz. The coupling factor of magnetic loop was obtained by multi-port transform theory and by using the energy attenuation method, and the size of the magnetic loop was determined. The thermal and structure analysis result shows that the peak in-put power can achieve 260 kW, with 50% duty factor for a single coupler, and 1 MW power can be achieved under four-channel coupler. After RF power test and conditioning, the RF power feeding to the cavity is reached to 484 kW, with 10% duty factor at present, and the power and duty cycle can be improved in future.