The transmission device with external power includes a first USB type-C connector, a second USB type-C connector, a power transmission cable, a data transmission cable and a switching unit. The first USB type-C connector and the second USB type-C connector are connected to the first electronic device and the second electronic device. The power transmission cable is connected to the first USB type-C connector and the second USB type-C connector, so that the first electronic device supplies power to the second electronic device through the power transmission cable according to charging information of the second electronic device. The switching unit is connected to a power input end and the second USB type-C connector through the power transmission cable. When the first electronic device supplies power to the second electronic device, the switching unit is turned on and the power input end supplies power to the second electronic device.