The present invention provides a positioning device for an optical membrane that includes at least one positioning hole formed on the optical membrane and at least one positioning pole interposed into the positioning hole respectively, the optical membrane is fixed onto a carrying substrate by traversing said positioning holes by said positioning poles, wherein the profile of each positioning hole extending in corresponding position in the expansion and shrink direction of the optical membrane. A certain width could be reserved in the positioning hole to allow the optical membrane expand and shrink freely at each fixing point and the positioning pole cannot interfere with the positioning hole at the moment of the optical membrane expands and shrinks to cause bending, thus to guarantee flatness of the optical membranes and improve the quality of the backlight module.