In the float glass defect inspection system based on machine vision, the common defects can be detected and classified correctly. However, the online glass ream inspection is still a problem to be solved, and the accuracy cannot fulfill the requirement of glass grading. As the glass ream is a rather slight optical distortion in most cases, it should be amplified so that it can be checked out easily. A Moiré method based on the theory of superposition shadow is proposed, which can magnify the distortion effectively without increasing the system complexity, and a STFT method is presented to analyze the distortion according to the characteristics of Moiré fringe pattern. With this method, the vision interference angle of glass ribbon can be calculated out accurately in real time, which makes it suitable for online glass ream inspection.