External gear pump working principle led to trapped oil phenomenon, direct impacting the life of the gear pump and working operational stability. The main reason of trapped oil phenomenon was briefly outlined, the main solutions to trapped oil phenomenon were introduced, the trapped oil volume was virtual measured by Pro/E, the bulk changes of trapped oil volume during gear pumps working was obtained. The phenomenon of the gear pump oil trapped could be eliminated through reasonable solving the changes of gear pump trapped oil volume. Based on this approach, dual gear asynchronous operation method was proposed, to solve the phenomenon of pump gear trapped oil.