An exemplary common voltage adjusting method for a liquid crystal display (LCD) (20) includes: providing a positive high level voltage to two sub-pixels of a first pixel unit and providing a negative high level voltage to the other sub-pixel of the first pixel unit in a first frame; inspecting the first pixel unit and generating a first color parameter; providing a negative high level voltage to the two sub-pixels of a second pixel unit and providing a positive high level voltage to the other sub-pixel of the second pixel unit in a second frame; inspecting the second pixel unit and generating a second color parameter; generating a common voltage adjusting parameter according to a comparison result of the first color parameter with the second color parameter; and adjusting a common voltage of the LCD according to the common voltage adjusting parameter for confirming a preferred common voltage.