Battery-casing sealing is the key factor for secure travel of new energy vehicles. We constructed a relatively accurate mechanical-simulation model by selecting a constitutive model, analyzing the influence of thermal elongation, verifying the grid-independence and comparing numerically by the pressure-measurement film on the basis of studying the physical performance of a certain type of sealing material that had been used in battery-casings after aging. Based on a porous-media model and combined with changes of macroscopic and microscopic contact characteristics of materials at different times after aging, the evolution rule of sealing performance with time was analyzed quantitatively by calculating the leakage. By analyzing the structure of circular arc bulge on the surface of sealing material, the radius of circular arc bulge with better sealing performance was obtained, which could reduce the leakage of sealing structure during the material’s lifecycle.