The gear pump is an important component of the hydrodynamic automatic transmission,which provides oil for the automatic shift system and lubrication and cooling system. Based on hydraulic automatic transmission design,according to the structural characteristics of the radial outer engaging in parallel multi-tooth gear pump,an internal study for trapped oil phenomena encountered in the process of their work,the causes and solutions are analyzed. Through gear pump structure optimization,the bias unloading groove design is proposed,and analysis of its size and location is carried out. Based on CFD,the three different forms of gear pump structure work flow field are analyzed and compared,and test bench is built for experimental study of the gear pump performance after the optimization design to verify the accuracy of the analysis. The results show that,when the bias unloading opening slot design appropriate parameters can effectively improve the phenomenon of trapped oil,gear pump and volumetric efficiency is improved. By comparing experimental test of volumetric efficiency shows accuracy and reliability of the structural optimization design and built mathematical model,a reference for such transmission gear pump design is provided.