Abstract: The operation condition of condensate gas gathering line is analyzed with HYSYS and PIPESYS software. The results show that the condensate gas is respectively gas-liquid two phase flow and single phase flow (gas phase)at lower pressure and higher pressure. However, when pressure continuously increases, the single phase flow and two phase flow simultaneously exist in the condensate gas, and the slug flow, at this time, comes into being. By means of the phase equilibria relations, the authors analyze the above phenomena, and point out that the original temperature of pipeline can be increased to avoid the slug flow to protect pipeline under the permissible operation condition.