Abstract: In order to solve the problem of coolant supply during the rotation of an airborne mechanical phase scan radar, a liquid rotary joint with leak detection function is designed. The joint adopts radially arranged structure of the annular flow channel and the low-compression double-layer sealing ring structure, and an interface for fiber optic cable is integrated. Because of the novel structure, the shape of the joint is ultra-thin, suitable for the compact phase scan radar. It has been verified by simulation and experiments that the joint has excellent performance such as small drive moment, long life, high reliability and stable performance and can provide a reliable liquid rotating connection function in the airborne environment for the radar. It can be widely used in mechanical phase scan radar.