Theoretical instantaneous flow of the linear conjugated internal gear pump was calculated based on the area sweep method, the corresponding relationship between the position of the meshing point and the instantaneous flow of the pump was obtained, and then the geometric flow ripple of the pump was calculated. The occurrence of trapped oil chamber is a common phenomenon during the actual operation process of the pump, and it is also a key factor affecting the smoothness of the pump outlet flow. The operation process of the linear conjugate internal gear pump was analyzed, the internal flow channel was divided into suction chamber, oil discharge chamber, gear intergrade chamber, gear ring intergrade chamber, and trapped oil chamber according to the control volume method. The AMESim simulation model of the linear conjugated internal gear pump was established, and analysis and simulation verification for the fluid movement status inside the pump were carried out. The results show that the model can reflect the instantaneous flow mutation caused by the occurrence of trapped oil chamber in the actual operation of the pump; the flow ripple rate of the simulation model is 2.29%, higher than the geometric flow ripple rate which is 1.71%.The research result reveals the cause and changing rules of the pump flow ripple, and provides a reference for the flow characteristics research and optimization design of linear conjugate internal gear pump.