A USB adapting circuit is suitable for being connected between a USB host and an external device. The USB adapting circuit includes a connecting port, a detecting circuit, a standby circuit and a main circuit. The standby circuit receives a power supply from the USB host and supplies the detecting circuit and the main circuit with the power supply. The detecting circuit is configured to output a connected signal when the external device is connected to the connecting port. The standby circuit outputs an enabling signal in response to the connected signal. The main circuit adapts between the USB host and the external device when receiving the enabling signal. Therefore, the main circuit does not work without receiving the enabling signal, and has a power saving effect.