Due to the depleted exploitation of shale gas reservoirs, liquid loading in horizontal wells during the later stages of exploitation is a common problem. Among these, the plunger lift has advantages such as a simple manufacturing process, low operating cost, and good performance, making it widely used in various gas fields. In view of the characteristics of plunger motion in the wellbore, researchers typically employ simulation and laboratory experiments to study both the plunger motion and gas-liquid flow mechanism. Existing simulations and experiments are generally conducted by setting either the plunger movement speed to be constant or the differential pressure between the wellhead and bottomhole to be constant. However, the wellhead pressure changes during the life cycle of gas production in a gas well, transitioning from the initial pressure state to a gradually decreasing pressure after the well is opened. Therefore, it is difficult for existing results to accurately reflect the dynamic coupling relationship between the plunger motion state and the differential pressure between the wellhead and bottomhole during the plunger lifting process. This paper establishes an adaptive simulation model of fluid-structure interaction (FSI) for plunger motion in gas well production, which accounts for decreasing wellhead pressure. And a model is established based on simulation results to calculate the liquid leakage upper of the plunger.