The invention relates to a fan rotating speed detection apparatus. The apparatus comprises a first 
capacitor, a first 
resistor, a first 
Schmitt trigger, a modulation circuit and a controller. One end of the first 
capacitor is connected to a 
signal output end of a fan, and the other end of the 
capacitor is connected to the input end of the first 
Schmitt trigger and is connected to the ground through the first 
resistor. The output end of the first 
Schmitt trigger is connected to the controller through the modulation circuit. A periodic 
signal output from the fan is converted into a pulse 
signal through a 
charge and discharge loop, the pulse signal is then output to the input end of the first Schmitt trigger, the first Schmitt trigger shapes the pulse signal into a rectangular wave signal, the modulation circuit modulates the rectangular wave signal and a reference 
voltage of the modulation circuit, then a 
voltage signal is obtained and input to the controller, the controller samples the 
voltage signal and a corresponding voltage value is obtained, and therefore the rotating speed of the fan is obtained. The apparatus can provides the program execution efficiency of the controller, and the detection cost for the rotating speed of a fan is reduced.