Abstract: In view of the growing need for water hydraulic actuators in underwater operations, a scheme was propounded to develop high performance water hydraulic axial piston motor for underwater rotary tools. In the water hydraulic system, the water hydraulic motor is the device that converts the pressure energy of water into rotating mechanical one to drive working mechanisms. Based on physicochemical properties of water and the working principle of axial piston motor, a physical model was developed, numerical and experimental studies were carried out to research its output performance, including the output torque, rotating speed and ripple. The results prove that the physical model is reasonable, and show the effects of the input pressure difference and the flow on the motor's output characteristics, which provide a basis for the development and improvement of the hydraulic motor.