With the development of LNG carriers, this paper focuses on the study of the influencing characteristics of sloshing in the coupled nonlinear ship motions. The aim is to develop the time domain prediction techniques un-der both regular and irregular wave conditions. The energy dissipation was modeled by changing the boundary condi-tion on tank solid boundaries. Under the linear free surface condition, to find out the potential, the fully nonlinear model was adopted for the inner tank flow and the boundary element model of Green function satisfying transient nonlinear body surface condition was used for external flow outside the tank. The boundary value problem was solved by the B-spline higher-order panel method. The ISITIMFB ( iterative semi implicit time integration method for floating bodies) was applied to solve the body’ s velocity and displacements. An extended principle to determine the dissipation coefficientμis extracted and the effectiveness of this method was demonstrated from the comparison results of the experimental and numerical situation.