The present invention provides a fan speed control method for avoiding inaccurate control caused by sudden changes in a power supply output state, including the steps of: acquiring an output current value of a power supply device; and then, determining a change of the output current value in a sampling timeframe, if a slope representing the change is positive, controlling the control signal generating unit to gradually replace the fan control signal currently output to the fan with another fan control signal on the basis of a first latency, and if the slope representing the change is negative, controlling the control signal generating unit to gradually replace the fan control signal currently output to the fan with another fan control signal on the basis of a second latency. The first latency and the second latency are determined by the output current value, respectively.