Explosive charges are usually encased by protective shells to ensure the safety of storage and transportation. However, the protective shell is often neglected in numerical simulations, which may affect the characteristics of underwater explosion (UNDEX) loads and structural dynamic responses. The present work adopts the Arbitrary Lagrangian-Eulerian (ALE) method in LS-DYNA software to simulate the damage characteristics of stiffened cylindrical shells subjected to shock wave generated by UNDEX. Numerical results obtained by cased and bare charges are compared, and the influence of the casing thickness is analyzed. The results indicates that the protective shell significantly reduces the amplitude of the shock wave. Compared with bare charges, plastic deformations of the front and back sides of the stiffened cylindrical shell are reduced by the cased charges, which indicates the existence of the protective shell can reduce the damage of stiffened cylindrical shells. Meanwhile, the thickness of the protective shell has negligible influence on the structural damage under the studied thickness range and standoff distance.