<p>It is crucial to improve the calculation efficiency of internal blast loads of a long-span spatial steel structure. This study develops an equivalent model for such loads using a one-way inclined single-layer cylindrical lattice shell structure as a case study. First, ANSYS/LS-DYNA was used to simulate free-air blasts and benchmark against experimental data, with peak overpressure errors below 8%, confirming the modeling approach and material parameters. Next, a numerical model of the cylindrical lattice shell structure under internal explosion was generated via the same modelling method and material parameters. The simulation results indicated that the internal explosion overpressure differed from that of free-air blasts, exhibited pronounced reflection and convergence effects, and was no longer related to the scaled distance. On this basis, an equivalent model combining a standard overpressure distribution with correction factors for reflection and convergence was formulated. Validation against two additional case studies demonstrated that the model provides conservative predictions, with average errors of 9.38% and 7.47% relative to detailed simulations. The proposed equivalent model therefore offers a rapid, reliable tool for the preliminary assessment of internal blast loads in similar spatial structures.</p>