Abstract: Through analysis and research,the paper makes out the mechanism of noise from outer-engaged gear pump:(1) the manufacturing accuracy of outer-engaged gear pump is not enough;(2) empty pumping in outer-engaged gear pump while turning with high speed or air entry in the oil tube;(3) oil remains in outer-engaged gear pump without relevant pressure-release trough;(4) the outer-engaged gear pump has too big changeable gear,which has less teeth, thus the pulsation rate for oil emission increases and noise from outer-engaged gear pump appears;(5) the pressure in the oil entry and exit is too big,the rapid increase of pressure causes considerable crash,which causes noise and vibration.According to the above-mentioned problems,the paper designs the special structure for eliminating the noise phenomenon,which not only lowers noise,but also enhances the volumetric flow rate of outer-engaged gear pump.It is a new approach to design high-quality and high-speed turning outer-engaged gear pump.