The large-scale explosive blast waves caused by a major explosion can result in overall structural damage, which is significantly different from the damage pattern caused by contact or near-field explosions. However, there is limited research on this topic. In order to study the dynamic response of steel beams under the action of long-duration shock waves, a large-scale numerical shock tube model was established based on the coupled Euler-Lagrange method. Then, the reflected overpressure time history acting on the steel beam was obtained using this numerical shock tube model. Finally, the influence of shock wave incident direction, duration, overpressure, and other parameters on the damage modes of the steel beam was studied using the Lagrangian dynamics method. The results show that the proposed numerical shock tube model can effectively simulate the diffraction phenomenon of long-duration explosive waves and provide reliable reflected overpressure and impulse time histories. With increasing incident angle of the shock wave, the displacement at the mid-span of the simply supported steel beam increases. When the peak overpressure of the shock waves is the same, with the increase in duration, the maximum displacement at the mid-span of the simply supported beam tends to increase, but the rate of increase in maximum displacement slows down. The simply supported steel beam is prone to flexural failure under the action of long-duration explosive shock waves, and locali buckling occurs in the flange plate at the mid-span of the beam.