1. A sensing method for a counterfeit bill detector, comprising steps of: using multiple image-sensing modules to scan multiple pieces of image information of a bill and converting the multiple pieces of image information into multiple values of digital image; comparing each value of digital image with a pre-stored image threshold value to generate a reference value; adding the multiple reference values to generate an image validation value; comparing the image validation value with a pre-stored validation threshold value to acquire a comparison result; and determining if the multiple pieces of image information are valid according to the comparison result.