Abstract: Natural gas has to reach certain dew point before it enters every household, which entails the dryness of natural gas transmission pipeline. The air drying process consists of two parts: air drying and water soaking, which are both complicated heat and mass transfer process. Based on the mass conservation, phase equilibrium and the Law of Fick, two parts' models are established. The equations in drying process are solved numerically and soaking process analytically. And the models are used for pipeline drying in West-East Gas Transmission Pipeline Project, which justifies the correctness of these models.