This paper investigates the dynamic response and structural performance of a reactor pit in a third-generation nuclear power plant under steam explosion loads through numerical simulation. A three-dimensional model of the internal reactor building structure was developed based, and LS-DYNA software was used to analyze the structural dynamics and assess the integrity under extreme loading conditions. The results indicate significant stress concentration on the inner wall of the reactor pit and at the connection between the pit and the base slab. Severe localized damage is observed in the groove areas on the inner side of the reactor pit and at the connection between the pit and the base slab. However, the extent of the damage is highly localized, and the deformation analysis reveals that the maximum displacement in the localized damage zones is approximately 2 cm. Then a post-blast gravity loading analysis is conducted, which indicates that the key vertical load-bearing components of the reactor building retain their integrity, ensuring no collapse or loss of structural capacity. This study confirms that the reactor building design can withstand high intensity steam explosion loads without compromising the safety and containment integrity. The findings provide valuable insights into the performance of nuclear plant structures under extreme dynamic loads, offering guidance for future design improvements and reinforcing the safety standards of nuclear power plants.