Abstract: This paper is aimed at improving the pressure-drop calculation method of gas-liquid two-phase flow in mobile pipeline and providing the theoretical basis for the design of gas-displacing-oil pipeline emptying. Firstly, the pressure-drop calculation methods of gas-liquid two-phase flow in horizontal pipe based on flow pattern were introduced. Secondly, the classification of flow pattern was reviewed and summarized and the existing problems related to the classification were pointed out. Thirdly, homogeneous flow model, drift flow model and separated flow model were compared and some typical or recently developed pressure drop calculation methods related to flow pattern were described. Fourthly, the common flow patterns and the evolutional trend of flow pattern and pressure drop in the process of gas displacing oil in mobile pipelines were analyzed. It is stratified flow at the initial stage of pipeline emptying, stratified flow and slug flow simultaneously at the intermediate stage and temporary annular flow at the final stage, and accordingly the pipeline is divided into pure gas section, gas-liquid mixed section and pure liquid section. Finally, the pressure-drop model for different operational modes was established and the urgent problems related to gas-displacing-oil mobile pipeline emptying were pointed out.