Abstract: There is the free surface in the main vessel of fast reactor. When long period earthquakes happen, the fluid will impact the wall of vessel and make the reactor dangerous. The 3D finite element model of the main vessel is constructed using ADINA, and the phenomenon of sloshing response to the free surface in the main vessel of fast reactor excited by 3 sine waves is simulated. The impact pressure from this research has provided important loadings for the integrality analysis of the main vessel. The feasibility of simulating the free surface flow and the advantage of dealing with FSI questions are validated.