A voltage detecting system is configured for detecting a voltage supplied to an electronic device. The voltage detecting system includes a voltage measuring circuit configured to measure the voltage, a voltage converting circuit electrically coupled to the voltage, and a voltage comparing circuit. The voltage measuring circuit has a first measuring range and a second measuring range that is wider than the first measuring range. The voltage converting circuit is capable of the converting the voltage to an output voltage according to a predetermined ratio. The voltage comparing circuit compares the output voltage with a reference voltage. If the output voltage is greater than the reference voltage, the voltage comparing circuit informs the voltage measuring circuit to select the second measuring range. If the output voltage is not greater than the reference voltage, the voltage comparing circuit informs the voltage measuring circuit to select the first measuring range.