Abstract: Yan'an Gas Field is located in a complex terrain, where the pipelines are multi-undulated with large elevation, and liquid accumulation in pipeline is one of the common problems in the production process of gas field gathering system. Specifically, the liquid accumulation in pipeline can easily cause slug flow and even affect the normal production of gas field. In this study, a calculation model of critical liquid-carrying gas velocity in the up-dip pipeline section was established based on the liquid film model, with consideration to the operating pressures, wellhead temperature, molar water content, pipe nominal diameter and pipeline inclination. In addition, the influence law of the largest inclination, the total elevation and the number of undulations of the multi-undulating pipeline on the critical liquid-carrying gas velocity was studied with reference to the operating data of Yan'an Gas Field. On this basis, the established critical liquid-carrying model of the up-dip pipeline section was modified. The modified critical liquid-carrying gas velocity model of the undulating pipeline had the calculation results in well agreement with the field data. The results indicate that the calculation results of the modified model have the accuracy up to 90%, and this model shows great adaptability for the multi-undulating surface gathering pipelines, which could provide theoretical support for liquid accumulation prediction in the multi-undulating pipelines of Yan'an Gas Field.