Zeolites are a class of solid acid catalysts with a wide range of applications in the current catalytic field due to their regular pore structure, large specific surface area, and efficient acidic properties. The catalytic reactivity of zeolites is closely related to their acidity. Accurately characterizing the acidic properties of zeolites is of great significance for establishing the structure-activity relationship between zeolite structure, acidity, and catalytic reaction performance. One of the most effective methods to characterize the type of acid centers and acid strength of zeolites is in situ infrared spectroscopy using pyridine as a probe molecule.First, this paper describes the experimental principles of in situ FTIR-pyridine adsorption method for characterizing the acidic properties of solid acid catalysts. Then, using ZSM-5 as a model, the in situ FTIR-pyridine adsorption was used to optimize the testing conditions for identifying the surface acidity of the zeolite and the effects of experimental conditions such as activation temperature, activation time, adsorption time of pyridine, desorption temperature and desorption time on the relevant FTIR characteristic peaks were investigated.The results showed that the optimal experimental conditions for the interaction between pyridine and acidic sites on zeolites were: activation temperature of 400 ℃, activation time of 60 min, adsorption time of 10 min at room temperature, desorption temperature of 150 ℃, and desorption time of 30 min. Under these experimental conditions,the pyridine was efficiently adsorbed with the acidic sites on the zeolites, and the intensities of the FTIR characteristic peaks corresponding to Brnsted and Lewis acids were saturated. Simultaneously, the interference of physical adsorption, hydrogen-bonded adsorption, and contaminants adhered to the sample surface on the adsorbed pyridine was effectively eliminated, and the optimal FTIR spectra were obtained with high repeatability. Finally, three modified zeolites, including Fe-ZSM-5, HZSM-5, and Na-HZSM-5, were characterized by optimized experimental methods, all of which obtained FTIR spectra with excellent quality, and the acid amount ratios of Brnsted acid to Lewis acid in agreement with the reports. The method improves the test efficiency and success rate. It excludes the relevant interference so that the measured information on the zeolites acidity category, strength, and relative content is more accurate and reliable. Meanwhile, the optimized experimental method provides a reference for characterizing the acidic properties of other solid acid catalysts, which is of great significance in guiding the preparation and mechanism research of solid acid catalysts.