Abstract: High-speed hydraulic pump aims to comply with the inevitable electrification trend of hydraulic power units. Among them, the internal gear pump can be expected in agricultural and engineering machinery, due to its simple structure, high power density, low noise, and small flow pulsation. However, the ever-increasing wear of friction pairs has limited the high-speed operation. The microscopic degrees of freedom of the pinion gear and the ring gear cannot be constrained under the spline or flat key connection. The extremely complicated kinematic and dynamic behaviors of the gear pairs can often occur under the inertia force, oil film pressure, and medium temperature rise. This study aims to explore the wear mechanism and lubrication conditions of the friction pairs in the high-speed internal gear pump. A transient dynamic model of the internal gear pump was then 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 the side plates 1 and 2, respectively. Specifically, the side plates were used to limit the axial displacement, whereas, 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 the surface-to-surface contact. The rest parts were defined as the general contact for their unknown contact state. The clearance of each friction part was determined, according to the actual product. The implicit dynamic analysis was used to solve the final state between each component. The contact pressure was used to represent the contact state of each component. The contacts then occurred, when the contact pressure was not zero. Furthermore, an in-situ measurement was carried out, 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. The micro-motion of the ring gear was clarified to combine the simulation and experiment, in which the axial micro-motion of the ring gear was verified by an in-situ measurement experiment. The results show that the ring gear concurrently shared the radial and axial micro-motion under the inertia force. The higher the rotational speed was, the greater the radial micro-motion of the ring gear was. There was also the random position of contact with the pump body. Moreover, the ring gear was biased to a certain side of the pump casing, and the contact frequency of the friction pair increased, although the axial micro-motion amplitude gradually decreased with the increasing speed. At the same time, the amplitude of the axial micromotion of the ring gear decreased by 27 μm, when the rotational speed increased from 1 006 to 3 002 r/min. But the frequency of contact with the mating surface increased by 1.46 times per second. There was a rebound behavior after contact and rebound again after contact between the end face of the ring gear and the mating surface of the pump casing. The different position of contact was then observed at the second time. The simulated and actual contact positions were highly compatible. The pair of gears wore significantly at the rotational speed of over 3 000 r/min. There was more significant wear on the end face of the ring gear in the flat key connection, indicating that the spline connection was more favorable at high rotational speed. No significant wear occurred on the pinion gear in either the flat key or spline connection, indicating that the axial micromotion of the pinion gear was less than that of the ring gear, with a maximum of 5 μm. Comprehensive tests and simulations show that the micro-motion behavior of the ring gear practically existed, where the rotational speed was one of the influencing factors. The friction vice clearance of the conventional rotational speed products cannot meet the high-speed operation. In addition, the influencing factors of the micro-motion of ring gear can greatly contribute to the anti-wear and friction reduction of the friction pair in the high-speed internal gear pump.