Before product natural gas enters NG transmission line, its dew point under the highest possible transmission pressure has to be lower than the lowest atmosphere temperature during transmission. However, there is residual water at internal wall and low-lying part before start-up, which would damp NG and thus causes it fail to meet the demand of low dew point. Therefore, the water needs to be removed from the line before start-up by certain process. Drying by dry air is a complicated heat and mass transfer process. Based on the Mass Conservation, Phase Equilibrium and the Law of Fick, the models of water drying process and closing-down process are established. The equations in water drying process are solved numerically, while closing-down process analytically. Relevant PC program has been developed as well. Then these models are used in West to East Pipeline drying, which justifies their correctness. In the end, some suggestions to design and operation are presented.