Rotary contactless power transmission couplers can be used for rotating ultrasonic machining equipment to provide power for highspeed rotating ultrasonic vibrators to replace the traditional contact carbon brush and slip rings.Such contactless power transmission method is safe and stable, and has no restriction on the speed. However, due to the gap between the primary and secondary magnetic cores, the leakage flux is large and the power transmission capacity is low. In order to improve the power transmission capability of the contactless rotary coupler, finite element simulation and experiments were used to optimize the design of the coupler by comparing and analyzing the magnetic flux distribution, coil selfinductance, mutual inductance between coils, coupling coefficient, coil AC resistance and other parameters of couplers with different materials and structures. The optimized contactless rotary coupler has higher coupling capacity and power transmission capacity.