Numerical simulation for the progressive collapse of the underground structure under its internal blast load has been reported in this paper. The three dimensional model of the underground structure with two-story and two-spans in both x-direction and z-direction covered with soil was built with explicit dynamic analytical software LS-DYNA. An improved method by considering the impact of the blast load was proposed in the simulation. Solid-fluid interaction algorithm, the erosion algorithm and stress initialization are used in the simulation. It is found that, compared with the alternative path method, the improved method gives more accurate predictions of the progressive collapse process. The improved direct simulation method is more cost-effective than the direct simulation method. The impact of the internal explosion can’t be ignored. The analysis may provide important information for the additional design guidance on progressive collapse of underground structures.