This paper analyzes the constant voltage and constant frequency starting method in detail, gives a detailed introduction to the constant voltage and constant frequency starting voltage and phase change frequency of the brushless DC motor with a 120° trapezoidal waveform of the back electromotive force, and analyzes the basic principle of constant voltage and constant frequency starting and the conditions that need to be met for realizing accelerated phase change. Combined with the mathematical model of the motor and the mechanical equations of motion, the motor is analyzed for each phase change of the renewed current phase and the conduction phase, and the calculation method of the constant voltage and constant frequency starting voltage range is deduced and the basis for determining the phase change frequency is given. The rotor motion equations and speed change equations are verified through simulation and compared with the actual rotor position and speed magnitude to verify the accuracy of the calculation results of constant voltage and constant frequency starting voltage and phase change frequency.