A method for controlling air volume including: 1) establishing a functional relation formula for air volume in a microprocessor control unit of a motor controller; 2) inputting a target air volume into the microprocessor control unit; 3) starting a motor by the motor controller to enable the motor to achieve a rotational speed and fall on a steady state; 4) recording the torque and rotational speed in the steady state, and calculating an air volume in the steady state; 5) comparing the target air volume with the calculated air volume; 6) re-recording a steady torque after the motor falls on a new steady state under an increased or reduced rotational speed, and recalculating the air volume in the new steady state; and 7) repeating steps 5) and 6) to adjust the rotational speed until the calculated air volume is equal or equivalent to the target air volume.