Abstract: The thickness of hydration film of clay particles is the theoretical basis of swelling mechanism of argillaceous expansive rock and soil. There are abundant data on the measurement of clay particle hydration film thickness, but the study of the measurement of clay particle hydration film thickness by atomic force microscopy is relatively rare, and the test method is not perfect. Based on the three-layer hydration membrane structure model and atomic force microscope test technology, through the test and research on montmorillonite powder, mudstone powder and mudstone rock slice, this paper puts forward the measurement method of the thickness of the hydration film, the preparation method of powder sample and rock sample, and the statistical processing method of the test data. The variation rules of the free water section, weakly bound water section, strong bound water section and clay particle section of the hydration film thickness test curve are summarized. Through comparative analyses with the existing research results, the rationality and feasibility of the testing the thickness of the hydration film of clay particles by the atomic force microscope is demonstrated. Combined with engineering practice, the engineering significance and theoretical value of the quantitatively obtaining the hydration film thickness in understanding the swelling mechanism of the argillaceous expansive rock and soil are discussed.