In order to research the dynamic response characteristics of underground cavity with different spans, numerical simulation of the vertical penetration of the projectile into the underground cavity was carried out by adopting small diameter bomb(SDB) and using ANSYS/ LS-DYNA finite element program. The abtained penetration depth was similar to the depth calculated by emprical formula. The research results showed that when the depth of the underground cavern was 5 meters, the stress at the vault was the largest, and the stress at cavity arch with small span(below 14 meters) was larger, while the stress at cavity spandrel with big span (above 24 meters) was larger. When the depth of the underground cavern was 10 meters, the stress at arch springing was large, the displacement response was the most severe. These parts were the weakness of structure, therefore strengthening measures should be taken in design.