A fan speed control device includes a management selection module, a first and a second buffers, a resistor, a transistor, and a switch. Depending upon whether a motherboard supports a system of a baseboard management control or not, the fan speed control device selectively uses an indicative signal of a serial input and output interface or an indicative signal of the baseboard management control to indicate a power-on status. At the power-on moment, the fan speed control device controls the fan to operate at its minimum speed, so that a system crash due to a power output shortage of a power supply can be avoided. As the system enters stable operation, the fan speed is controlled by a pulse width modulation signal.