Based on the assumption of the ideal fluid, the coupled dynamics equationsof movement of rigid tank and nonlinear sloshing of liquid are establishedthrough H-O principle, with the modified potential function introduced todescribe the moving boundary of rigid tank and fluid. Galerkin's method isused to discrete the equations. Benchmarked with some FEM and BEM results,feasibility of the present method is proved. The responses of the coupledsystem to the excitement by some forces are simulated. The equivalent theoryis used to explain the increase of the natural frequencies.