[Abstract] A viscous two phase flow model based on the Volume of Fluid (VOF) method and Immersed Boundary Method (IBM) is used in this research to numerically simulate sloshing problem occurred in the LNG tank (prismatic tank).The whole computational domain is rectangular, and VOF method is used for tracking and updating the free surface, immersed boundary method for boundary processing. A block up coefficient is used for the processing of volume fraction F so that VOF can process the bevel edge boundary. Finally we achieve the whole process of the sloshing tank motion, and compare the liquid surface height achieved by different methods; this study has reference value to this type of sloshing motion.