A USB interface system capable of automatically adjusting connection speed and power consumption capability and a method thereof are provided. The method includes configuring a slave device to perform a first handshake procedure with a main device, and communicate with the main device by using a first connection specification; detecting a first power-off event by using a slave power detection module; when the first power-off event occurs, recording first power-off information by the memory unit. If the slave device is re-connected to the main device, the slave power detection module is configured to perform a second handshake process with the main device, and determine to re-communicate with the main device in a second connection specification different from the first connection specification according to the first power-off information.