Abstract: High-speed hydraulic pump is to comply with the inevitable electrification trend of hydraulic power unit, the internal gear pump, because of its simple structure, high power density, low noise, small flow pulsation and other advantages, is widely sought-after in agricultural machinery, engineering machinery and other mobile machinery. However, increased wear of the friction pair is one of the bottlenecks limiting its high-speed design. Under the spline or flat key connection, the microscopic degrees of freedom of both the pinion gear and the ring gear are not constrained, which leads to extremely complicated kinematic and dynamic behaviors of the gear pair under the influence of inertia force, oil film pressure and medium temperature rise. In order to explore the wear mechanism of the friction pair of high-speed internal gear pump and improve the lubrication conditions of the friction pair, a transient dynamic model of internal gear pump was established. The casing of a real internal gear pump cartridge kit was simplified as a circular ring, and the low and the high pressure side plates were simplified as side plate 1 and side plate 2, respectively, where the side plates were used to limit the axial displacement and the radial displacement of the ring gear was limited by the circular ring. The inner spline surface of the pinion gear and the spline shaft, the pinion tooth surface and the inner tooth surface of the ring gear were defined as surface-to-surface contact, and the other parts were defined as general contact for their unknown contact state. The clearance of each friction part is determined according to the actual product, and the implicit dynamic analysis step was used to solve the final state between each component. The contact pressure was used to characterize the contact state of each component, and when the contact pressure is not zero, contact has occurred. Further, an in-situ measurement experiment system was established, where three eddy current displacement sensors were arranged on the same axis to monitor the spokes of the ring gear, the ring gear teeth and the pinion gear teeth, respectively. The micro-motion characteristics of the ring gear were clarified by combining simulation and experiment, in which the axial micro-motion of ring gear was verified by in-situ measurement experiment. The results show that, under the influence of inertia force, the ring gear exists radial and axial micro-motion phenomenon at the same time. The higher the rotational speed, the greater the radial micro-motion of the ring gear, and the contact position with the pump body has a randomness. Although the axial micro-motion amplitude gradually decreases with increasing speed, the ring gear will be biased to a certain side of the pump casing, and the contact frequency of the friction pair is increased. By increasing the rotational speed from 1006 r/min to 3002 r/min, the amplitude of the axial micromotion of the ring gear decreases by 27 μm, but the frequency of contact with the mating surface increases by 1.46 times per second. There is a phenomenon of rebound after contact and rebound again after contact between the end face of ring gear and the mating surface of pump casing, and the contact position is different at the second time. The simulated and actual contact positions are highly compatible. When the rotational speed exceeds 3000 r/min, the gear pair wears, and the wear on the end face of the ring gear is more significant in the flat key connection, indicating that the spline connection is more favorable at high rotational speed. No significant wear occurs on the pinion gear in either the flat key or spline connection, indicating that the axial micromotion of the pinion gear is less than that of the ring gear, with a maximum micromotion of 5 μm. Comprehensive test and simulation results show that the micro-motion phenomenon of the ring gear is practically existed, and the rotational speed is one of the factors affecting its micro-motion, and the friction vice clearance design of the conventional rotational speed products can not meet the high speed operation. In addition, further clarification of the influence factors of micro-motion of ring gear is the key to the design of anti-wear and friction reduction of the friction pair of high speed internal gear pump.