Abstract: The high accuracy calculation method for an isolated system is the prerequisite of carrying out its analysis and design. In this paper, the FBS method, which has been widely used in the automobile industry to calculate the dynamic characteristics of ship isolation systems, is introduced. Aiming at the defects of the present FBS theory (which assumes the perfect condition of force balance and velocity continuity and only considers the stiffness and damping characteristics of the isolator but ignores the influence of the mass), a research is performed to deduce the calculation formula for floating raft isolation systems based on the matrix of the isolator impedance, which eliminates the errors brought by the equivalent isolator characteristics theoretically. The model test indicates that the refined method has higher accuracy at high frequency, which satisfies the need of engineering applications.