A method for controlling a fan speed of an electronic apparatus and the electronic apparatus using the same are provided. The method includes: detecting an input voltage, an output current, and a working temperature of the electronic apparatus; calculating a target speed of the fan according to the input voltage, the output current, and the working temperature; providing a speed control signal indicating the target speed to drive the fan and obtaining a fan speed signal indicating an actual speed of the fan; and performing a close-loop control based on the fan speed signal, so that the actual speed is adjusted to the target speed.