A system is used to measure a fan rotational speed. The system includes a main controller, a first socket coupled to the linear fan, a second socket connected to the main controller, and an optical fiber amplifier connected to the main controller. A PWM (pulse width module) fan is coupled to the second socket, the PWM fan sends a first rotational speed signal, which represent a rotation speed of the PWM fan, to the main controller via the second socket. The optical fiber amplifier is capable of radiating light on fan blades of the linear fan, and sensing light reflected by the fan blades to count a rotational speed of the linear fan, and generating a second rotational speed signal which represents a rotational speed of the linear fan. The optical fiber amplifier sends the second rotational speed signal to the main controller.