A switch short-circuited diagnosis method includes steps of: determining an initial voltage interval of multiple voltage intervals according to voltage relationships between voltages of a first phase wire, a second phase wire, and a third phase wire; performing a switch short-circuited diagnosis of a first bidirectional switch module in the three consecutive voltage intervals from the initial voltage interval, and including steps of: turning on a first switch branch, a second switch branch, or a third switch branch of the first bidirectional switch module according to the voltage relationships between the voltages of the first, second and third phase wires, determining whether an overcurrent occurs to diagnose whether the first switch branch, the second switch branch, or the third switch branch of the first bidirectional switch module is in a short-circuited state, and performing the switch short-circuited diagnosis for the next voltage interval.