To explore the destructive forms of bridge damage caused by explosions of hazardous materials in vehicles and the distribution of explosion load pressures on the bridge deck, a refined numerical model was established using AUTODYN software. The study analyzed the regional distribution characteristics of the bridge load pressure field under various explosion conditions with different shapes and sizes of steel plates, and determined the critical dimensions at which the steel plates play a blocking role against shock waves. In response to the challenges of conducting bridge explosion experiments, which involve high risks and large expenses, the research referred to a detailed inspection report of a real bridge after an explosion accident and inferred its explosion damage process. Based on the least squares method, a polynomial curve fitting was applied to numerous of numerical calculation results, and the traditional calculation formula for explosion shock wave pressure in the free air domain was modified. A prediction formula for the peak overpressure on the bridge deck under the explosive effects from the vehicle-borne cargo, taking into account the blocking of carriage steel plates, was proposed. The load pressure distribution calculated by this formula corresponded well to the damaged areas on the bridge deck as reported in the real bridge inspection report.