Abstract: To solve the problem of post disaster assessment of bridges after accidental explosion, terrorist attack and weapon strike during service, the damage and damage evaluation of a real single box three chamber reinforced concrete box girder structure under explosion load were carried out by using ANSYS/LS-DYNA finite element software, and its damage and failure modes under different explosion positions and explosive equivalent were analyzed. It is found that under 100 kg and 200 kg explosive equivalent, the reinforced concrete box girder was mainly damaged locally, the damage of a single box chamber was large, and the overall damage of the box girder was limited. The explosion above the box girder web and inside the box chamber caused serious damage to the box girder. The damage evaluation method of box girder based on the section residual moment of inertia was proposed. It is suggested that the damage and failure of box girder structure under explosion load can be divided into four levels: mild damage, moderate damage, severe damage and complete damage. The damage and failure of box girder structure under explosion load were evaluated.