Abstract: The flow characteristics can be expected to precisely characterize in the crescent internal gear pump. In this study, the mathematical models were developed for the internal leakage and viscous friction loss of the crescent internal gear pump, according to the hydrostatic support oil film theory and Newton's friction theorem. A CFD simulation model with the two-phase flow and dynamic grid was constructed to analyze the flow characteristics of the gas containing oil using the structural parameters and real flow characteristics of the gear pump. A comparison was made on the simulation and theoretical calculations. A series of experiments were carried out to verify the model. The results demonstrate that the continuous periodic variations were observed in the instantaneous volume flow rate and instantaneous input power curves in the 1/3 cycle. Four pulsations of both curves were also found to commensurate with the number of gear pair meshing during this time. 60.11% between the theoretical and simulation values was found for the total leakage flow, and 66.67% was for the total viscous friction power loss. Once the streamline was parallel in the static zone in the laminar particle flow, there was the entirely turbulent in the moving region. The shear flow also moved clockwise at a speed of more than 12 m/s along the outer walls of the outer gear and the inner gear ring, while the differential pressure flow moved counterclockwise at a speed of more than 12 m/s along the inner and outer walls of the crescent diaphragm. Several scale vortices were observed between the fully sealed teeth. The fluid velocity inside the vortex was zero, due to the liquid dropping off near the core. The flow was stopped off at the minimum clearance under the sealing effect of the oil layer on the meshing gear surface, where the maximum leakage in the meshing area was 0.16 L/min. The deviation rate was 1.36% for the volumetric efficiency difference between the test and simulation. By contrast, the deviation rate was 1.73% for the overall efficiency difference, indicating the acceptable error range. As such, a precise mathematical model was proposed for the flow characteristics of crescent internal gear pump. The applicability of the numerical calculation model was validated to assess the accuracy of the simulation. These findings can be used as a guideline to enhance the design theory and examine the internal flow field.