The invention discloses a method and terminal device for improving video signal definition. The method includes: acquiring information of a connected display device; selecting a corresponding definition improving algorithm according to the acquired information of the display device to process video output signals, and finally outputting the processed video signals to the display device for display. By adopting the above technical scheme, the problem in the prior art of fuzzy or unclear display images caused by mismatching between output images of the mobile device and the connected display device as a uniform fixed algorithm is adopted to magnify images when a mobile device is connected to a display device for video play is solved.