A power source circuit is connected to a power source and a digital subscriber line (DSL) modem. The power source circuit includes a boosting converter, a storage capacitor, a voltage converter, a first switch, a second switch, and a trigger. The boosting converter boosts an input voltage of the power source. The storage capacitor charges by the boosted voltage. The voltage converter converts the input voltage of the power source into a rated working voltage of the DSL modem. The trigger triggers the first switch to electrically connect the storage capacitor to the voltage converter and the second switch to electrically connect the power source to the voltage converter.