Abstract: Far-field shock waves induced by large-scale powerful explosions are characterized by long positive pressure duration, large impulse and high dynamic pressure. Under the action of such long-duration explosion waves, structures are not only subjected to overpressure, but the effect of dynamic pressure (explosion-induced wind) should also be taken into consideration. Columnar structures and members with rectangular cross-sections are widely applied in engineering practice, yet their load characteristics and flow field features under the action of long-duration explosion waves remain unclear. Aiming at the diffraction problem of long-duration explosion waves around rectangular columns, this paper adopts a numerical simulation method, incorporates the influences of positive pressure duration (<italic>t_d</italic> = 50 ms~2000 ms) and the width-to-height ratio of rectangular columns (<italic>B</italic>/<italic>D</italic> = 0.5~2.5), investigates the variation law of the drag force of rectangular columns and the time history of their impulse, analyzes the distribution characteristics of reflected overpressure on the surface of rectangular columns, and clarifies the evolution process and interaction mechanism of explosion waves and flow field structures. The research results show that the drag coefficient of rectangular columns differs significantly from the empirical values in the field of wind engineering, and it is less affected by the width-to-height ratio but more significantly influenced by the positive pressure duration. With the increase of positive pressure duration, the drag coefficient of rectangular columns under all width-to-height ratio conditions shows a variation trend of decreasing first, then increasing, and finally tending to be stable. The flow field structure around the rectangular column exhibits diverse configurations under different positive pressure durations, evolving from a symmetric wake to a distinctly asymmetric distribution as the duration increases.