The invention relates to an image input and output device with the screen display generation and a method thereof; the device comprises an input interface, an output interface, a first screen display mixing unit, a second screen display mixing unit and an image synchronization unit. A single on-screen display (OSD) control unit is applied in the OSD generation method of the image simultaneous input and output device for simplifying the OSD generation costs of the device, thereby allowing the input or the output images to have the consistent horizontal and vertical synchronization time standards, achieving the simultaneous sharing of the single OSD control unit by the device and allowing output and input ends to have the different OSD contents.