Abstract: A fan speed control includes a power adapter, a CPU programmed with a phase splitting program, a current-limit resistor, a capacitor, a TRIAC, and a switch module, the CPU having a signal input terminal connected in series to the power input terminals of the power adapter through the current-limit resistor to obtain synchronous signal frequency as input signal, a signal output terminal connected in series to the capacitor and then the gate of the TRIAC for enabling the CPU to control triggering of the TRIAC, the TRIAC having the gate connected in series to the capacitor, and the anode and cathode respectively connected to the fan motor and the power output terminal of the power adapter, so that the fan motor, the TRIAC and the power adapter form a loop in which the TRIAC controls the operation speed of the fan motor; the switch module having one end connected to the control input terminal of the CPU for controlling the output state of the signal output terminal of the CPU and the other end connected to the po