Abstract: In the natural state, the constraints of soils are between completely drained and undrained conditions. Due to the influences of the probe size, penetration velocity, soil properties and other factors, the partial drainage conditions will occur during the indentation of piezocone penetration test (CPTU). In order to study the influences of the drainage conditions on the CPTU data, the triaxial compression tests are carried out under different partial drainage conditions. The modified Cam-clay (MCC) model considering the asymptotic state is established by introducing the strain increment ratio. Then the cylindrical cavity expansion theory is derived, and the mechanical transformation model for the CPTU data is proposed. The relationship between the normalized penetration velocity and the drainage coefficient is studied through the numerical simulations and model chamber tests. The relationship between the strain increment ratio and the drainage coefficient is obtained by combining the CPTU data with the mechanical transformation model. Finally, based on the CPTU data of the Suzhou River Deep Tunnel Reservoir Project in Shanghai, the correctness of the mechanical transformation model is verified. The results show that the MCC model considering the asymptotic state can describe the stress-strain relationship of soils and reasonably explain the mechanical mechanism of the CPTU penetration under the partial drainage conditions.