Flow pulsation is the fundamental cause of the vibration of the gear pump and the generation of fluid noise. In order to obtain the flow pulsation characteristics, a certain type of straight line conjugate internal meshing gear pump was taken as the research object, and the theoretical instantaneous flow curves corresponding to different coincidence degrees was analyzed based on MATLAB software, and the change rule of the gear pair geometric parameters on the flow pulsation rate was studied. The results show that the gear pair withdraws from the meshing and the theoretical instantaneous flow is the minimum. When the pulsation rate increases, the influence degree of the geometric parameters is arranged from large to small: gear reference circle radius,gear tooth tip circle radius, transmission ratio (difference between two teeth), ring gear tooth tip circle radius; when decreasing, it is arranged from large to small: the number of gear teeth,gear reference circle radius,gear tooth profile half angle. For the transmission ratio that meets the calculation, when the gear tooth number is determined, the ring gear tooth number takes a smaller value; when the ring gear tooth number is determined, the gear tooth number takes a larger value; the tooth profile half angle takes a larger value; the gear and ring gear tooth tip circle radiuses take smaller value; the gear reference circle radius is within 33.5~35 mm.