For releasing trapped-oil pressure in high-speed external gear pumps, a new relief groove with main ring-shaped profile and inner end-wedge is invented for larger relief area and simpler processing. Firstly, based on the relief principle of small backlash, the structure and dimension of new groove were given. Secondly, by the three dimensional (3D) model’s area measurement, all dynamic relief areas of some different meshing position were measured in a trapped-oil period. Thirdly, based on the derived and verified trapped-oil pressure model, the trapped-oil pressures were executed respectively in the new groove and conventional rectangle-shaped and conventional circular-shaped at normal 3 000 r/min, and only in the new groove at high 6 000 r/min and ultra-high 9 000 r/min. All results indicate that whole relief capacity of the new groove is strong, among them, the relief area surrounded by main ring-shaped profile of new groove is respectively increased by 31% and 165% compared with that of rectangle-shaped groove and circular-shaped groove, and similar linear characteristics of the changed relief area surrounded by inner end wedge-shaped structure of the new groove is perfectly matched to relief requirements near in minimum trapped-oil position. The respectively added percent of trapped-oil peak pressure with discharge pressure at normal and high and ultra-high speed is 2% and 77% and 14% in the new groove, so it’s trapped-oil impact is very little. The trapped-oil valley pressures respectively is 0.03 MPa and -0.22 MPa and -0.61 MPa at normal and high and ultra-high speed, so, it’s cavitation is regarded as nothing at high speed and something at ultra-high speed, fortunately, if the new groove cooperated with large backlash relief, the cavitation is also regarded as nothing at ultra-high speed. It is concluded that the new relief groove is excellent for trapped-oil relief at high-speed and it’s processing is easier than conventional relief groove.