Interference experiment of polarized light is important in college physics experiment teaching. Based on Mach-Zehnder optical path, a self-built optical path is improved to observe the interference phenomenon of polarized light, to measure the change of light intensity, to judge the polarization form of polarized light, to calculate the refractive index of air in the laboratory environment. Data processing and error analysis are completed to having an error within 5%. Compared with the traditional experimental teaching method, the improved experimental device not only has clear and intuitive principle, but also can be expanded and applied by adding some optical components, so as to improve the efficiency of college physics experiment teaching and mobilize the enthusiasm of students to participate in experiments.