A method for controlling air volume of an electric device to be constant, the device being adapted to exhaust or supply air, the method including: A) establishing M constant air volume control functions Qi=F(n) corresponding to M air volume points CFMi in the microprocessor of the system controller; B) allowing the microprocessor to receive or preset a target air volume IN-CFM; C) starting the motor, when the motor operates in a stable state, comparing M air volume points CFMi with the target air volume IN-CFM, and ensuring that the target air volume IN-CFM falls within two known air volume points CFMi and CFMi−1; D) using the two known air volume points CFMi and CFMi−1 to calculate a constant air volume control function Q0=F(n) corresponding to the target air volume IN-CFM by interpolation method; and E) controlling a motor parameter Q0 and a rotational speed n.