Abstract: Medium in the blow-down pipeline of high-pressure gas trunk lines is non-steady flow, its hydraulic calculation should not adopt Darcy formula which is applicable to normal pipelines of pump station. For this reason, based on Fano equation, a program has been made according to fluid mechanics and basic flow theory of gas pipeline, which can calculate blow-down volume, cumulative blow-down time, pressure and temperature of section and gas storage volume at any moment. Comparison of the program results and graphical result shows the error is less than 2%, besides, the program calculation method makes up the defect of graphical method that only instant blow-down volume can be calculated. Based on blow-down system calculation cases, major factors affecting hydraulic characteristics of blow-down pipeline, such as pipeline length, diameter, wall roughness and gas temperature, pressure have been analyzed. The results show that the length and diameter of blow-down pipe affect blow-down system stronger than other factors. The study can provide a reference to the design of blow-down pipeline of gas trunk lines.