Abstract: Investigating the methods for interpreting piezocone penetration test (CPTU) data is of great significance in improving the accuracy and reliability of geotechnical investigation and analysis. In this study, an interpretation method for CPTU in cohesive soils based on undrained cavity expansion theory is proposed and verified by comparison with a group of 37 calibration chamber tests in kaolin clay. The results show that the proposed method can provide predictions of CPTU data. The calculated CPTU data is consistent with the calibration chamber tests, and the predicted results for cohesive soils are consistent with the traditional soil behaviour type (SBT) charts. Additionally, this study investigates the effects of overconsolidation ratio and initial state parameters on the normalized cone tip resistance and the pore pressure ratio, and obtains fitting formulae for kaolin clay. Finally, the relationships between the normalized effective cone resistance and overconsolidation ratio and initial state parameters are theoretically derived. A CPTU interpretation curve for kaolin clay is plotted on the SBT charts, providing a certain theoretical basis for interpreting in-situ testing data in geotechnical investigation.