Abstract: The gasification at the negative pressure is an important unsteady flow phenomenon in the pipeline. As there is gas phase in the pipeline, it is more difficult to analyze the operation situation and control the pipeline. Based on the mathematical model established gasification at the negative pressure for the section of the pipeline, the authors have done some tests and theoretical studies in the aspect of the volume of gasification, its law of the change, gas absorption, gas bubble vanishing and so on, when pressure transient in the pipeline after the gasification at the negative pressure. Simulation calculation shows that the mathematical model and the method conform to the situation of the pressure and flow fluctuate as the gasification at the negative pressure better than the concentrated void model. It is the first time that the balance gasification curve is used in describing the gasification process at the negative pressure.