Abstract: Due to the leakage flow, secondary flow and other unstable flow phenomena often exist in the external gear pump under high viscosity conditions, which have a great impact on the operation of the gear pump. In order to more accurately explore the characteristics of these unstable vorticity, the flow characteristics of unstable vorticity in the external gear pump was analyzed. The results show that the increase of viscosity can improve the flow state in the gear pump, but the high pressure and low pressure areas in the gear pump increase obviously, and the outlet flow decreases gradually. The greater the viscosity, the longer it takes for the pump outlet flow to stabilize. Within a cycle, the inlet position of the oil suction chamber changes from no vortex to the appearance of vortexes of equal size and opposite rotation direction, to the two vortexes becoming larger and finally falling off into several small vortexes. From the timeT₀+1/5T, the larger vortex appears in the gear meshing and the oil discharge cavity, and an obvious "horseshoe" vortex is formed in the oil discharge cavity. The larger the viscosity is, the less the vortex strength of the gear pump becomes at the same position in a cycle.