The invention relates to a seaborne boundary condition simulation method for an FLNG device liquidation process. The method is characterized by comprising the following steps of: (1) establishing a three-dimensional control volume unit, working mediums of which flow in an FLNG liquidation process system, under a sea condition, and carrying out analysis to obtain a momentum equation of working medium flow directions; (2) obtaining a three-dimensional motion equation of a hull or a platform on the basis of sea condition basic data of an actual operation sea area; (3) carrying out calculation to obtain a translational motion equation and an angular motion equation of working medium gravity centers; (4) establishing a one-dimensional fluid flow equation for representing flow, in the FLNG liquidation process system, of the working mediums under the actual sea condition; and (5) calculating sea condition boundary condition parameters according to the one-dimensional fluid flow equation, and loading the sea condition boundary condition parameters into a FLNG device liquidation process dynamic model so as to realize application of actual sea condition environment to FLNG device liquidation process. The method can be widely used for improving the simulation facticity and emergency working condition prediction correctness of the FLNG device liquidation process.