This paper provides the rational explanation of influence of liquid sloshing on the FLNG motion using spring-mass type mechanical model to represent the liquid cargo behavior and the coupling effects on the FLNG motion. The mechanical model of sloshing was originally developed in the mid-1960s and used to represent the coupling effects on the spacecraft motion for predicting and controlling the spacecraft behavior and stability. Authors believe that the mechanical model is a useful tool for better understanding the interaction of liquid cargo and the floating body motion. Firstly, the concept and associated formula of the mechanical model are explained and the parameters of the mechanical model for a rectangular tank are obtained analytically. Secondly, the equation of floating body motion and the mechanical model are coupled. Next, the numerical demonstration is performed and compared to the experiment and the calculation by other means. Finally, a rational explanation of the influence of liquid sloshing on the FLNG motion is provided.