Abstract: Most of the existing hydrate prediction models can not describe the formation process of hydrate slug in gas pipelines. We proposed a physical model for the hydrate formation state in gas pipelines based on distributional characteristics of gas and liquid in pipelines, established a mathematical model to predict the formation of hydrate slug in gas pipelines by means of the heat-transfer, multiphase-flow and phase-equilibrium theories, deduced definite conditions to solve the related problems, and finally described the formation process of hydrate slug by numerical simulation. The results show that the model can forecast the formation position, configuration and changing process of hydrate slug successfully. The gas flow rate, and the thickness and the thermal conductivity of thermal-insulating materials can significantly affect the initial formation region of hydrate. With the increase of gas flow rates, the liquid-carrying capacity of gas cores will get better and the thickness of liquid film will reduce. Furthermore, a series of changes in parameters will take place during the formation of gas hydrate slug, such as the increase of pipeline pressure differences, the decrease of temperatures, the enhancement of liquid-carrying capacity of gas cores and the decrease of liquid film thickness, all of these may, in return, react on the further formation of hydrate slug.