Abstract: MachZehnder interferometer is suitable for studing the density change of gas. The variation of refractive index makes the beams getting through gas have different optical paths. Therefore, the density of gas can been acquired by changing the interference arm to affect interference fringe patterns. In real measurement,the fringe patterns generated by MachZehnder interferometer are collected by frame frabber and CCD, then the fringe spaces are processed by a computer to get the gas density change. This paper is base on light interference theory, analyses the property of MachZehnder fringe patterns and establish a math model. According to the math model, an arithmetic was designed to deal with MachZehnder interference fringe patterns. The arithmetic includes image preprocess(i.e. noise extraction of image)，gray image binarization and fringe pattern sharpening.