The invention discloses a method for measuring airflow of a fan motor. The method is characterized by comprising steps as follows: A), M constant airflow control functions Qi=F(n) corresponding to M airflow points CFMi are established in a microprocessor of a motor controller, wherein Q can be input power, direct current bus current or torque, and n is the rotating speed of the motor; B), the motor is started, the real-time rotating speed n0 and the parameter Q0 of the motor are detected after the motor is in a stable state, the Q0 is put into a plurality of constant airflow control functions Qi=F(n) corresponding to a plurality of airflow points CFMi, a plurality of rotating speeds ni are obtained, the real-time rotating speed n0 and a plurality of corresponding rotating speeds ni are compared, and the real-time detection airflow CFM0 is ensured to be located between CFMi and CFM(i-1); and C), the real-time detection CFM0 is calculated with an interpolation method. According to the method, blocking of an air filter net can be accurately judged, a signal can be output for giving an alarm, the mathematic model is simple, the control is reliable, and the accuracy is high.