A circuit for alarming abnormal state of a computer fan includes a detection circuit coupled to the computer fan, a controller coupled to the detection circuit and the computer fan, an alarm unit coupled to the controller. The detection circuit detects a power source and a speed control signal received by the computer fan, the controller determines the work state of the computer fan according to the power source, the speed control signal, and a speed signal received from the computer fan and raises an alarm through the alarm unit if the computer fan is not working normally.