The blast load is very important to the construction of underground rock engineering and the defense engineering project. The study on underground structural failure modes under blast loading is always a hot problem. The underground structure is developing to big span direction. But so far, the study of big-span deep underground structure's dynamic response is seldom. The structural mechanics under blast loading is still not known well. So，the nonlinear dynamic finite element procedure of ANSYS/LS-DYNA is used and the fluid-solid coupling algorithm is selected to study the dynamic response of underground structure with different spans. The explosion is closed and the detonating distances are changed. The analysis shows that the mechanics status of big-span is complete changed compared to the small-span one. And also the maximum stress curves of structures with different spans are obtained.