Abstract: This paper presents the principle of water draining process after field hydrotest for self-constructed new oil/gas pipelines. For simulation of such water draining, the testing system-the first one in China-is designed and constructed in the laboratory. The system provides an experimental tool to simulate how the drainage pressure are affected by relevant parameters like inlet pressures of different buffer tank, flowrates, drainage pipe diameters and air contents in liquid. For the purpose of accuracy, the test system, including all lines and related facilities, is checked before hydrotest and water draining test. In addition, high-speed camera is deployed to record conditions of fluids and pigs in pipelines. Test results show that pressure impulses may be observed in terminals of pipelines during the two-phase pigging tests by using gases to discharge water.