The invention discloses a generator intelligent constant speed system based on demand speed measurement and control. The system is characterized by comprising a photoelectric encoder for collecting anelectrical pulse signal of a generator rotor; a generator rotor revolution number acquisition unit for obtaining data of revolutions of the generator rotor corresponding to the time according to thereceived real-time pulse signal of the photoelectric encoder and storing the data in a memory; a processing unit for calculating the demand speed according to the received real-time pulse signal of the photoelectric encoder and sending a throttle/valve opening control command; a display unit for displaying the demand speed calculated by the processing unit in real time; the memory used for storingthe data of the revolutions of the generator rotor corresponding to the time. The system adopts a slip frequency method to measure the average speed and is high in speed measurement and control accuracy and response speed and capable of achieving accurate response to the speed.