In this paper， the explosion damage assessment of prestressed concrete box girder is studied. First， the numerical analysis model of rectangular beam explosion is conducted， and the accuracy and reliability of the explosion numerical model is verified by the field test results. Then， based on the residual bearing capacity of the structure， the damage assessment criterion of box girder at explosion load is established. By analyzing the residual bearing capacity of box girders at explosion load， the P-I （pressure–impulse） curves of various damage degrees of box girders are determined. The results show that the concrete strength has a greater influence on the critical value of overpressure and impulse， while the yield strength of reinforcement has less influence. With the increase of damage grade， the influence amplitude of each parameter on P-I curve of box girder is different. Finally， the formula fitting of P-I curves of all damage grades of box girder with different concrete strength is performed， and the fitting parameter β is analyzed.