Abstract: The displacement method of long distance gas pipeline always use instrument to control the process in the field. The "gas-to-gas process", the common way, is considered as being short of theory base. The authors put forward the "gas-to-gas process" mathematic model with three fluid dynamic equations and diffusing equation, and make use of the difference equation and develop special software. Taking the west pipeline segment of West to East Gas Transmission Pipeline as a target pipeline, a calculation is made and the result fits the fact well, which can be used in pipeline project analysis. The model provides a theoretical and technical support for the long distance gas pipeline's commissioning.