A touch input method and a device thereof are disclosed. The touch input device includes a display module for displaying an image, a backlight module for emitting a light source, at least one light-sensing unit for sensing a reflected light of the light source reflected by at least one object on the display module and generating a light intensity signal correspondingly, and a processing module for receiving the light intensity signal and determining based on the light intensity signal at least one touch point on the display module touched by the object. The touch input method is applicable to the touch input device and includes the steps of using the processing module to receive the image, using the processing module to analyze the image and obtain an image analysis result, and regulating a light intensity of an invisible light emitted from the backlight module based on the image analysis result.