Unloading groove design is one of the most important in the design of gear pump parts, and the design is reasonable or not directly related to the working performance and the service life of the gear pump. Based on the research of CBG type gear pump as an object, using CFD technology flow field analysis, the effects were researched of unloading groove structure parameters and the uncontrollable factors on trapped oil pressure. The results show that more different unloading groove structure parameters are gotten for the influence of trapped oil pressure, combined consideration is in need of the unloading groove structure parameters and environmental performance parameters, which provide theoretic basis for the optimization design of the structure of the unloading groove.