Abstract: During the pigging of a natural gas pipeline in hilly areas, the running speed of pig tends to fluctuate violently due to the influence of topographic relief and liquid loading in the pipeline. The pig collides with the pipeline at the elbow and generates impact load, which threatens the safe operation of the pipeline. In this paper, the hilly pipeline section between Longling Gas Transmission Station and Baoshan Gas Transmission Station of China-Myanmar Natural Gas Pipeline was taken as an example. Based on the pipeline simulation technology, the relationships between the maximum running speed of the pig and the pressure, throughput and liquid loading amount of the pipeline were analyzed. It is indicated that the liquid loading amount is the dominant factor influencing the maximum allowable running speed of the pig. In addition, the maximum allowable running speed of the pig under the constraint that the pipeline stress shall not be higher than the material's allowable stress and its corresponding maximum allowable liquid loading amount are determined. Finally, the safe pigging condition characterized by pipeline throughput and pressure difference was presented by correlating the liquid loading amount with the pipeline throughput and the pressure difference between two terminals. It provides a reliable and practical method for discriminating the safe pigging conditions of hilly gas pipelines.