For the key problems of high noise and vibration of high-speed wheel reducer used in electric vehicle, a dynamic model of the wheel reducer is developed and the mesh characteristics and dynamic characteristics are investigated under rated power, maximum speed and maximum torque load conditions. Then the amplitude of structural noise is calculated and analyzed. Also the effect of contact ratio on the dynamic performance of the system is evaluated and macro geometry parameters of the transmission system are optimized by using MASTA. Results show that the peak-to-peak values of the transmission errors for the output gears are relatively higher and the main structural noise comes from the high-speed input shaft. Compared with the design with contact ratio as a non-integer value, the design with an integer contact ratio leads to a lower changing ratio of total length of contact lines and the contact load fluctuation, the changing ratio of the mesh stiffness and the structural noise from the bearings supporting locations are decreased obviously. The optimization of macro parameters makes the dynamic performance of gear pairs better.