Abstract: For a floating structure with multiple sloshing tanks, the structure motion, external hydrodynamics and sloshing dynamics of liquid tanks are mutually determined with complex coupling mechanism. This study introduces an effective time-domain decoupling algorithm for an accurate motion simulation of floating structure with multiple sloshing tanks. The algorithm is derived based on the modal decomposition approach. By decomposing the external hydrodynamic force and nonlinear sloshing forces in each liquid tank of the floating structure, this study gives a time-domain decoupling motion equation. With this algorithm, the instantaneous acceleration of a floating structure at any instant is calculated explicitly without iterations. Limitations of the conventional iterative method in terms of the iteration number, truncation errors and numerical convergences can be avoided. The CPU time consumption on dealing with the coupling effects can be greatly reduced. Combined with the boundary element method, the algorithm is applied to time-domain simulations of a floating structure with either a single liquid tank or multiple tanks. For single-tank cases, the time-domain decoupling algorithm is validated by comparing with the experimental measurements. This study first analyzes effects of the sloshing dynamics on a single-tank floating structure. A specific frequency range is found, outside which the floating structure shows a steady motion in the time domain. For lower wave frequency cases around this range, the sloshing force and external wave force can be in anti-phase or even cancelled, so that the motion of the structure is weakened. For higher wave frequency cases, the sloshing force can be in the same phase with the external wave force, and the liquid sloshing can eventually amplify the structure motion. Further, a floating structure with four liquid tanks is further investigated. It shows that the nonlinear sloshing forces can affect the surge and pitch motion of the structure, but with little effect on the heave motion.