HPR1000 adopt In-vessl Retention (IVR) to mitigate severe accidents. When the IVR strategy fails, a steam explosion may occur, which can impose additional dynamic impact loads on the internal structure (I.S.) of the reactor building and threaten its capacity. Therefore, it is necessary to research the possible damage of the structure. This paper uses numerical simulation methods to study the structural performance under steam explosion. First, the actual structure geometry is appropriately simplified, a finite element model is established, and suitable material constitutive models and boundary conditions are selected based on the structural stress characteristics. Secondly, the loading method of the model is selected based on the characteristics of the steam explosion load. Finally, the response of the structure is obtained through nonlinear dynamic analysis, and the damage degree and integrity of the structure are evaluated. The analysis results show that the reactor pit structure has a certain degree of damage, but the damage range is small, and local damage does not affect the integrity of the overall structure.