Fiber-optic Mech-Zehnder interferometer can be used to multiplex and demultiplex wavelengths. The ΔL and Δφ directly influence the properties of M-Z. Through lengthening the ΔL, the density of wavelength dividing can be improved. The longer the ΔL, the more sensitive to temperature the M-Z. The method for making the M-Z work stably is explored. The wavelength separation is 0.8 nm, and the extinction raio is high.