Abstract: In order to meet the requirements of the diameter-variable structure of composite pipe strings in offshore gas wells, solve the problem of liquid accumulation in gas wells, and extend the production life of gas wells, a new diameter-variable plunger structure was designed. Force acting on the diameter-variable plunger during diameter enlargement and reduction caused by collision was analyzed. In addition, the plunger drainage and gas production process was simulated, and tests were carried out, such as laboratory diameter-changing behavior observation, measurement of liquid lifting capacity and intake pressure, and analysis of liquid lifting efficiency of the diameter-variable plunger. The research results show that the diameter-variable plunger can run smoothly in the diameter-variable pipe string, and the intake pressure required for diameter enlargement is proportional to the liquid lifting capacity. The diameter-variable sealing structure has excellent liquid lifting efficiency. The research results can provide an experimental basis and theoretical reference for the application of diameter-variable plungers in offshore gas wells.