A power supply apparatus for string light includes a power cable, a switching circuit, and a power transformer. The switching circuit is electrically connected to the two wires and configured to selectively form a voltage grading between the two wires. The power transformer includes a power transforming circuit and a control circuit. The power transforming circuit receives an external power via an input terminal, transforms the external power into a driving power, and outputs the driving power via an output terminal. The receiving end of the power cable is electrically connected to the output terminal. The control circuit is electrically connected to the power transforming circuit and the two wires. The control circuit detects a voltage grading status between the two wires, and generates a switch signal according to the voltage grading status to the power transforming circuit to modulate the driving power.