A video signal conversion method, a corresponding video signal conversion device and a display system including the video signal conversion device, by which low-resolution video signals can be converted and combined into a high-resolution video signal, such that a high-resolution image combined by low-resolution images can be displayed on an ultra-high definition display screen. The video signal conversion method includes: receiving in parallel a plurality of sub-frames divided from a low-resolution image of a video signal; performing image processing on each of the received sub-frames; and synthesizing the plurality of sub-frames after being subjected to the image processing into a high-resolution image frame and displaying the high-resolution image on a display device.