When foundations, such as caissons, spudcans, and mud mats, are pulled out from the saturated soft clay foundation, negative excess pore pressure will develop in the interface between the base and the soil. This phenomenon will significantly increase the uplift force. Therefore, estimating the developed negative excess pore pressure in the interface under unloading conditions is important. The development of excess pore pressure was investigated based on the triaxial extension test under isotropic consolidation condition. The method of using the modified Cambridge model to predict the excess pore pressure was programmed and has been proven to be feasible. By associating Henkel's theory in p-q space, the generation mechanism of negative pore pressure was revealed. The research indicates that excess pore pressure is initially negative and then becomes positive in triaxial extension test under isotropic consolidation. The generated excess pore pressure can be divided into two parts. The first part generated from the change in the mean total stress is negative, whereas the second part generated from the change in deviatoric stress is positive. However, these two parts are unbalanced. Research results provide theoretical basis for the development of excess pore pressure in soft clay to analyze the pull-out of structure and the excavation of foundation pit.