A video displaying device connected to a first display comprises a connecting port, a second display, a signal processing unit, a microcontroller, and a switching unit. The connecting port is used for connecting to the first display, the signal processing unit is used for generating a first video signal, the microcontroller is used for generating a control signal and transmitting the control signal to the signal processing unit for generating a second video signal, and the switching unit is used for triggering the signal processing unit to selectively transmit the first video signal and the second video signal to the connecting port and the second display.