The characteristics of gas⁃liquid two⁃phase flow during the pigging process is helpful to the design and operation of the pipeline itself and the upstream and downstream equipment. In this paper, the multiphase flow simulation software OLGA is used to study the gas⁃liquid two⁃phase flow law during the process of natural gas pipeline pigging. The changes of key analysis pipeline such as flow pattern, pressure, liquid holdup, pigging velocity, fluid volume and other parameters. The results show that: the pigging process in the undulating terrain, the gas⁃liquid two⁃phase stratified flow, slug flow and annular flow pattern. The pressure at the entrance of the pipeline is the largest, and the pressure decreases gradually along with the pressure fluctuation; the overall speed of the pig is not fluctuating, the average speed is about 3.5~3.8 m/s. The maximum length of liquid slug in the process of pigging is 3 851.0 m. The liquid holdup is constantly changing, the maximum clear liquid is close to 300 m3. The research conclusion has certain practical significance to the pigging operation of the natural gas pipeline in the relief area.