A fan speed testing system for testing a testing fan includes a testing fan, a collecting module, a processing module, a converting module; and a displaying module. A label is attached to the testing fan and capable of rotating with the testing fan. The collecting module is configured to collect light reflected off of the label as the fan rotates in order to produce a pulse signal. The processing module is configured to adjust the pulse signal to be a standard square wave. The converting module is configured to obtain a fan speed data of the testing fan according to conversion times between high levels and low levels of the standard square wave. The displaying module is configured to display the fan speed data.