Aiming at the problem of electromagnetic vibration and noise of motor, it was found that improving the sinusoidal characteristic of air gap flux density does not necessarily reduce the vibration and noise of motor. In view of this, the order and frequency of the main force wave causing the vibration and noise of the motor were analyzed, and the specific functional relationship between the third order harmonic of air gap flux density and the vibration and noise of the motor was explored. In order to analyze the order and frequency of the main force wave which caused the vibration and noise of the motor, the force wave table was eliminated based on the analytical method, and the main noise source was determined. In order to explore the influence of the third order harmonic of air gap flux density on motor vibration and noise, the mathematical models of air gap flux density and radial force wave were established, and the optimal amplitude of the third order harmonic of air gap flux density was solved by genetic algorithm. The analysis results show that improving the sinusoidal air gap flux density does not necessarily reduce the vibration noise of the motor, and the third order harmonic of air gap flux density has an optimal value in a certain range to minimize the vibration noise of the motor. The analysis results provide a reference for the design of low noise motor.