A power adaptor is provided. The power adaptor includes a voltage converter, a connecting port, a first transformer and a controller. The voltage converter receives an input voltage and determines whether to convert the input voltage to an output voltage according to an indicating signal. The first transformer includes a primary side and a secondary side. The primary side and the secondary side are coupled to each other, and a first end and a second end of the secondary side are coupled to the connecting port, respectively. The controller generates the indicating signal according to a voltage at the primary side of the first transformer. The connecting port is used to connect to an electrical device, and when the connecting port is electrically connected to the electrical device, a first end and a second end of the secondary side are short to a reference ground end of the secondary side.