The gearbox whine noise seriously decreases the driving comfort of an electric passenger car. A 6-degree of freedom(DOF)dynamic model of a helical gear system was built, and the whine noise generation mechanism was analyzed. The root cause of the problem was found through NVH tests of the gearbox and vehicle. A rigid-elastic coupling finite element model of the reducer assem- bly was built. The accuracy of the model was validated by a modal test. The structure-borne noise of the reducer in full acceleration condi- tion was predicted using the acoustic-structure and the rigid-elastic coupling model. Gear parameters such as pressure angle, helix an- gle, contact ratio etc. were studied to determine their influences on the whine noise. Gear tooth Micro-geometry modification parameters were optimized to reduce the transmission error of the first meshing gears. The whine noise of the gearbox was finally eliminated.