In order to detect the pH value of liquid easily, the pH sensitive sol-gel film was coated by chemical etching in the corrosion area after stripping the multi-mode optical fiber. Then a Mach-Zehnder interferometer pH detection sensor based on core mismatch was fabricated. Spectrometer was used to detect the response spectrum of the sensing structure to pH sample solution, and the response spectrum data were analyzed. The results show that when the pH value of the sample solution is in the range of 4.02~10.01, the wavelength shift of the response spectrum and the change of the pH value show a good linear law. The center wavelength of the detected trough drifts to short wave direction by 567 pm for each unit of pH change. At the same time, the sensing structure has good mechanical strength after coating, and can be better applied to the detection of liquid pH.