Abstract: Water accumulation in the low-lying areas of undulate pipelines will seriously threaten the operation of pipelines. Herein, the optimal pigging scheme for the initial pigging stage of the large undulate pipelines in mountainous areas was studied by test, and the calculation method for the volume of residual water in the low-lying area was proposed. The straight plate pig should be adopted in the initial pigging stage. Definitely, the optimum speed range of the straight plate pig should be 3.6-7.2 km/h, and the interference should be 4%-5%. Specifically, the water leakage of pigs was analyzed by changing the characteristics of pipelines, and thus the influence rules of different slope length, gradient, weld and elbows on water leakage were obtained. On this basis, the calculation method for the residual water volume and the average water film thickness on the pipeline wall in the low-lying areas was put forward through theoretical analysis. The research results could to provide reference and support for the subsequent deep pigging and drying operations.