A parallel power supply includes a built-in test switch and a control and determination unit. The built-in test switch is arranged to generate a first detection signal. The control and determination unit is coupled to the built-in test switch. When receiving the first detection signal, the control and determination unit is operative for enabling a detection mechanism according to the first detection signal in order to detect an operation of the parallel power supply, and accordingly generating a detection result. A power detection method for a parallel power supply includes: disposing a built-in test switch inside the parallel power supply; and when receiving a first detection signal generated from the built-in test switch, enabling a detection mechanism according to the first detection signal in order to detect an operation of the parallel power supply, and accordingly generating a detection result.