Abstract: The leakage of radioactive, flammable and other dangerous gases from the pressure-bearing shell would pose a major threat to the environment, and the leakage assessment is a necessary prerequisite for the optimization of emergency measures. Aiming at the micro-cracking of the thick shell wall under severe accidents, an efficient and quantitative analysis numerical model for the leakage rate is established. Based on the principle of flow conservation, an improved model is proposed for calculating the leakage rate through a variable cross-section channel formed by multi-segment micro-cracks in a thick-walled structure. The concrete plastic damage model is applied to simulate the nonlinear damage of the structure under its ultimate load, and the model for calculating the scale of smeared micro-cracks is improved by the transformation between the geometric irregular element and the regular space realized through an isoparametric function. Through numerical examples, the research results were compared with those in the literature. The stability and validity of the above two sub-models are verified separately, and the feasibility of the model is further verified by the leakage rate analysis of the damaged shear wall. Finally, the model is applied to a prestressed concrete complex pressure-bearing shell resisting high temperature and internal pressure damage. The results show that the improved model is suitable for the leakage rate analysis of thick-walled, porous and, other complex pressure-bearing structures, which has a certain practical engineering significance.