Abstract: Water hammer is a sort of phenomenon of unsteady flow. It often occurs in oil pipeline system. When the pressure in pipe has changes actually caused by hydraulically transient flow, if the boiloff does not exist at some high place where the pressure is not lower than the vapor pressure of oil, the flow in pipe is single-phase flow; otherwise, a gas-liquid two-phase flow will appear in the local pipe, which makes the water hammer analysis more difficult. Thus, it is necessary and important to analyze not only single-phase but also two-phase flow. In this way, the flow laws can be truly reflected, and all kinds of running statuses can be roundly simulated in pipeline system to ensure its safe and optimum operation.