Sloshing behaviors in liquid containers have been thoroughly studied for several decades by both experiments and numerical simulations. However, very few efforts are devoted to address the performance of gas pretreatment system of FLNG (floating liquefied natural gas) plants, particularly for the liquid entrainment estimation in the gas phase of multiphase separators. In this work, a multiscale simulation method was performed to study the sloshing impact on gas pretreatment for FLNG plants. First, computational fluid dynamics (CFD) simulations were performed to quantify liquid entrainment for oil-gas separator under sloshing. Next, CFD results were utilized by the gas pretreatment process modeling, where flow maldistribution in the oil-gas separator and absorption column due to sloshing was coupled with steady-state process simulations Results show that the sloshing behavior increases the amount of liquid carryover into the gas phase substantially, while the performance of the gas pretreatment system decreases consequently.