A background image updating method is adapted to a touch screen including a first and a second image modules. A first and a second images are captured by the first and the second image modules as a first and a second background images, respectively. (a) Whether an absolute value of a brightness difference between a third image captured by the first image module and the first background image is greater than a predetermine value is compared. (b) If the result of the step(a) is yes, whether an absolute value of a brightness difference between a fourth image captured by the second image module and the second background image is greater than another predetermine value is compared. (c) If the result of the step(b) is yes, a fifth and a sixth images are captured by the first and the second image modules as the first and the second background images, respectively.