Abstract: Based on the Galerkin/Least squares finite element method under arbitrary Lagrange-Euler description,the equilibrium iteration algorithm in which forces applied by fluids onto rigid bodies are treated as converging parameter is established for analyzing the fluid-solid interaction problems,and a new algorithm of mesh updating on the basis of pseudo elasticity method is proposed which can maintain fluid elements sides straight with mid-nodes coinciding with the center of element sides during mesh motion.The numerical example is about a mass-spring system vibrating freely in closed fluids which is analyzed with the method proposed,and the equivalent damping ratio,added-mass introduced by fluid-solid interaction are obtained as well as the streamlines of flow and pressure contours.