Abstract: In view of the problem that the existing swing hydraulic motor can only output one torque and one rotation speed, a new type of double-stator swing hydraulic motor is put forward, which combines the structure of vane swing hydraulic motor and the thought of double-stator. This motor consists of a rotor, an inner-stator and an outer-stator to form 2 groups of motors in one case. This motor can achieve a variety of connections, such as inner motors working alone, outer motors working alone, inner and outer motors working together, and inner and outer motors working differentially, so this motor can realize multistage speed and multistage torque output through changing its connections. In order to accurately analyze the leakage and volumetric efficiency of the double-stator swing hydraulic motor, and to obtain some reasonable gap seal size and seal structure improvement proposals. Through the analysis of the internal structure of double-stator swing hydraulic motor, the geometric displacement calculation formulas of inner and outer motors are summed up and the main internal leakage paths are analyzed; the main internal leakage paths consist of end face clearance and radial clearance leakage. A general formula of the total leakage is obtained through establishing the flow's mathematical expression of every leakage path. The total leakage of this motor is not only related to its internal structure, but also related to the speed and pressure. The volumetric efficiency of this swing hydraulic motor in different connections is calculated theoretically. Every leakage of inner and outer motors is calculated theoretically, and the end face clearance leakage of motors is relatively bigger than the radial clearance leakage. The end face clearance leakage of outer motor accounts for 94% of the total leakage of outer motor, and the end face clearance leakage of inner motor accounts for 73% of the total leakage of inner motor. The maximum and average leakages of the double-stator swing hydraulic motor are calculated theoretically. When the travel time of this motor is 3 s, and the pressure of the ports is 10 MPa, the maximum theoretical volumetric efficiency of this motor is 96.58%, and the minimum theoretical volumetric efficiency is 66.83%. In view of the end face clearance leakage, a seal structure optimization program is put forward. It can improve the volumetric efficiency of this motor through adding the floating side panels at the both sides of this motor. The improved and unimproved prototypes of double-stator swing hydraulic motors are tested in the experimental platform, and the volumetric efficiency of the motor prototype in 4 different connections is tested. It turns out that with the increasing of differential pressure of the motor ports, the volumetric efficiency decreases, and the volumetric efficiency of this swing hydraulic motor in different connections is different; when the differential pressure of the ports is fixed, the volumetric efficiency of this motor in outer motor working alone is the highest, and the volumetric efficiency of this motor when inner and outer motors working in differential connection is the lowest. When the travel time of this motor is 3 s, and the pressure of the ports is 4 MPa, the volumetric efficiency of this motor in inner motor working alone is 88.81%, the volumetric efficiency of this motor in outer motor working alone is 92%, the volumetric efficiency of this motor in inner and outer motors working differentially is 86%, and the volumetric efficiency of this motor in inner and outer motors working together is 90.32%. The reason why the volumetric efficiency of this swing hydraulic motor is lower in differential working is that its leakage increases and the theoretical flow decreases in differential working. Because the processing accuracy of the experiment prototype is lower, there is a certain deviation between the experimental results and theoretical analysis, but they are basically the same. The improvement of the motor end seal can increase its volumetric efficiency by about 11%. This result can provide reference for the design and application of double-stator swing hydraulic motor.