Abstract: Venting of natural gas pipeline is a complex process with pressure, temperature and gas velocity varying with time. To correctly calculate the venting time of long-distance natural gas pipeline, the unsteady partial differential equations are established for venting lines. Then, these partial differential equations are transferred to total differential equations by characteristic line equation and solved to get the venting time of the natural gas pipeline. The calculation results of this method agree well with the venting line data in engineering practice, which indicates that this method can correctly calculate the venting time of natural gas pipeline.