Abstract: In order to improve the volumetric efficiency of low speed high torque water hydraulic motor, the flow distribution performance of port plate pair is analyzed quantitatively. Taking the port plate pair of low speed high torque water hydraulic motor as the main research object, the mathematical models of the leakage flow and power loss of the end face of port plate and rotor are established by the equilibrium equation and the flow equation. Due to the irregularities of middle seal belt, the leakage flow is not calculated by using the existing equations. It needs to be approximately divided into 5 parts. Then based on the pressure-flow equation, the leakage flow formulas of the end face of port plate are derived in detail. Whereafter, according to the principle of hydrostatic pressure bearing, the working character of port plate is calculated and analyzed. The water film thickness, medium temperature and rotating speed of port plate and rotor can be the performance indices of the port plate pair. And the effects of clearance, temperature and rotating speed on flow distribution performance of water hydraulic motor are analyzed in different supply flow ways. The research shows that when the clearance is increased, the leakage flow and the power loss of port plate and rotor are increased. And the higher the temperature of water, the more the power loss of port plate. Meanwhile, when the inner ring is working, the performance of low speed high torque water hydraulic motor is better than that when the outer ring is working. Consequently, the clearance and water temperature should be decreased in order to reduce the leakage flow and the power loss of port plate pair and improve the volumetric efficiency and the properties of water hydraulic motor. But the water film can't decrease unlimitedly. Taking friction, wear, lubrication and manufacture into consideration, the water film thickness should be controlled at about 4-6 μm. Due to the increase of power loss, with water temperature increasing, the low speed high torque water hydraulic motor does not work for a long time under the condition of higher temperature. When all things are considered, the water temperature should be controlled at (20±5)℃. Meanwhile, the principle prototype of low speed high torque water hydraulic motor is designed and made on the basis of these studies. To analyze the characteristics of prototype, no-load and loading test are respectively carried out. Based on the experiment, the corresponding performance curves are obtained. The experimental results show that developing low speed high torque water hydraulic motor is feasible, with its minimum pressure of 0.7 MPa at no-load. When the load is 45 N·m and the rotating speed is 150 r/min, the maximum pressure is 2.9 MPa. As the torque increases, the mechanical efficiency will increase accordingly, but the volumetric efficiency will decrease. Meanwhile, the mechanical and volumetric efficiency of water hydraulic motor are greatly improved with the increase of revolving speed. In the experiments with a load, the volumetric and mechanical efficiencies of principle prototype of water hydraulic motor are 90.97% and 93.59%, respectively. Furthermore, the principle and feasibility are verified for the developed low speed high torque water hydraulic motor by the experiment, as well as the correctness of the study results. Moreover, the theory of design and the key technical problems of low speed high torque water hydraulic motor are solved. This research lays a good base for further development in the future. And the research work can also improve the converting efficiency, prolong the no-failure lifetime of key parts, reveal failure mechanism and establish new design theories and methods of water hydraulic motor. Therefore, the study on this project has not only important theory significance, but also a vast application prospect.