A power supply apparatus (1) for string light (400) includes a power cable (100), a switching circuit (200), a power transformer (300), and a casing (331). The switching circuit (200) is electrically connected to two wires (112) of the power cable (100) to selectively form a voltage grading between the two wires (112). The power transformer (300) includes a power transforming circuit (310) and a control circuit (320). The power transforming circuit (310) receives an external power (Ve) via an input terminal (311), transforms the external power (Ve) into a driving power (Vout), and outputs the driving power (Vout) via an output terminal (312). The receiving end (101) of the power cable (100) is electrically connected to the output terminal (312). The control circuit (320) detects a voltage grading status between the two wires (112), and generates a switch signal to the power transforming circuit (310) to modulate the driving power (Vout). The power transforming circuit (310) and the control circuit (320) are disposed within the casing (331) and the power cable (100) is disposed outside the casing (331).