Abstract: The penetration resistance of cone penetration test (CPT) in layered soil is significantly affected by the soil interface, but there is still a lack of systematic explanation for the phenomenon of "leading and lagging" when the probe crosses the soil interface. The division of soil layers through the penetration resistance still depends on engineering experience. A series of transparent soil model tests are carried out to simulate the penetration process of the probe in layered soil. The penetration mechanism of CPT in the layered soil is studied by observing the resistance curve of the probe and the deformation of soil near the probe. Combining with the cavity expansion theory and the Mohr Coulomb criterion, the method for calculating the influence depth of the cone resistance is proposed. Based on the test data and theoretical calculation, it is found that the influence range of the plastic zone in the two-layered soil with the upper sand and the lower clay is consistent with the "leading" depth of the penetration resistance curve. PFC is used to analyze the influence factors of the interface effct reflected by the CPT penetration resistance in layered soil. The results show that the soil deformation during CPT penetration is related to the soil strength, the initial in-situ stress and the position of soil interface. The strength difference between two adjacent layers of soil has a significant influence on the "leading and lagging" depth. Based on the research results, the soil layer is divided according to the plastic zone of soil and the leading depth of soil. Compared with the current complex methods of soil layer division, the proposed method can improve the efficiency and accuracy of soil layer division.