Abstract: In the undulating terrain area, liquid loading may be formed in the pipelines in the low-lying area due to the precipitation of water vapor and condensate in the pipelines, which may block the pipelines and reduce the efficiency of gas transportation, further threatening the safety of the pipelines and equipment. Therefore, the critical liquid-carrying gas velocity in updip pipelines was studied with reference to the actual production data. It is found that the pressure drop of updip pipeline decreases at first and then increases with the increase of superficial gas velocity. Regarding the minimum pressure drop point as the critical state of liquid carrying, the superficial gas velocity is the critical liquid-carrying gas velocity. As shown in the studies on change rule of the critical liquid-carrying gas velocity under different dip angles, water content and pipeline diameter, the critical liquid-carrying gas velocity changes logarithmically with the increase of dip angle. Besides, it also increases linearly with the increasing of water content and pipeline diameter. By obtaining the critical liquid-carrying gas velocity formula in combination with the liquid film model hypothesis, the prediction effect of the formula is simulated and verified with the field data.