Abstract: In the process of water drainage after the hydrotest of long-distance gas pipeline, pressure pulse may occur in complicated undulated pipe sections, and consequently results in safety accidents. To study the influences of liquid level in downward inclined pipe sections and gas injection pressure on the maximum pressure pulse induced in the process of pigging, a water drainage system for the hydrotest of undulated pipelines was designed independently in this paper. Then, combined with the relevant variables, the corresponding test schemes were prepared, and some solutions were put forward correspondingly to address the operational problems encountered in the tests (e.g. pig wearing and ball sticking). In the tests, the backward movement of pigs was captured by using the high-speed camera for the first time, and the causes of this phenomenon were analyzed. It is conducive to the improvement of mathematical models related with pigs. It is indicated that the maximum hydraulic pulse increases with the ascending of liquid level in downward inclined pipe section and the increasing of gas injection pressure. The test study on water drainage process after hydrotest of great-fall gas pipeline provides guidance for pipeline commissioning and safe operation.