A fan speed testing system includes a voltage regulating module, a rotational frequency collecting module, and a rotational frequency converting module. The voltage regulating module receives an AC voltage signal and converts the AC voltage signal to a variable first DC voltage signal to adjust a rotational speed of a fan. The rotational frequency collecting module detects the rotational speed of the fan and outputs rotational frequency signals. The rotational frequency converting module receives the rotational frequency signals and determines the rotational speed of the fan according to numbers and period of time level voltage switching in the rotational frequency signals.