A background subtraction method implemented by an image module, wherein the image module is disposed under a fingerprint sensing region of a touch screen of an optical fingerprint identification system, and the background subtraction method comprises: recording at least one first image of a first object, at least one second image of a second object and a target image, wherein the first object has a uniform color and has a first reflectivity, the second object has a uniform color and has a second reflectivity, and the first reflectivity and the second reflectivity are different, wherein the first object covers the image module when the at least one first image is captured and the at least one second object covers the image module when the at least one second image is captured; generating a first background image by calculating an average of the at least one first image so as to remove noise in the first image; generating a second background image by calculating an average of the at least one second image so as to remove noise in the second image; dividing a plurality of first pixel values of the first background image to the power of one or more by a plurality of second pixel values of the second background image to the power of one or more to obtain a plurality of relative values of the first background image relative to the second background image to obtain a mask image; subtracting the second background image from the target image to obtain a first background-removed image; and calculating and outputting a second background-removed image, wherein a first mask-multiplied image is obtained by multiplying the first background-removed image by the mask image, wherein a compensated image is calculated to be positively proportional to the first mask-multiplied image, and wherein the second background-removed image is calculated by adding the first background-removed image and the compensated image. recording at least one first image of a first object, at least one second image of a second object and a target image, wherein the first object has a uniform color and has a first reflectivity, the second object has a uniform color and has a second reflectivity, and the first reflectivity and the second reflectivity are different, wherein the first object covers the image module when the at least one first image is captured and the at least one second object covers the image module when the at least one second image is captured; generating a first background image by calculating an average of the at least one first image so as to remove noise in the first image; generating a second background image by calculating an average of the at least one second image so as to remove noise in the second image; dividing a plurality of first pixel values of the first background image to the power of one or more by a plurality of second pixel values of the second background image to the power of one or more to obtain a plurality of relative values of the first background image relative to the second background image to obtain a mask image; subtracting the second background image from the target image to obtain a first background-removed image; and calculating and outputting a second background-removed image, wherein a first mask-multiplied image is obtained by multiplying the first background-removed image by the mask image, wherein a compensated image is calculated to be positively proportional to the first mask-multiplied image, and wherein the second background-removed image is calculated by adding the first background-removed image and the compensated image.