Abstract: A model for the flexible rectangular cylinder vibrating in water was presented to simulate the fluid-structure dynamic interaction. First, the dynamic equation for the water-cylinder dynamic interaction under earthquake action was developed by finite element method. The water-cylinder interaction was represented by an added mass matrix, which was fully populated. However, the fully populated added mass matrix was difficult to be implemented in the commercial software. Then, a simplified model that used the lumped added mass matrix corresponding to the rigid motion and flexible motion to replace the fully added mass matrix was presented. It should be noted that the added mass caused by flexible motion was the product of the lumped added mass matrix and a correction factor. Based on the curve fitting method, the simplified formulas of the correction factor under different length-width ratio relevant to width-depth ratios were eproposed. Finally, the simplified formulas for the earthquake induced hydrodynamic forces on rigid rectangular cylinder were proposed by curve fitting method.