Abstract: The theoretical calculation methods and numerical simulation software of vacuum pipeline conductance value under different flow regimes are quite different,and the applicability and accuracy of different methods and software are still lacking in systematic research. This paper takes the sampling pipeline as the research object and uses the average Knudsen number to divide the pipeline into three flow regimes:continuous flow,transition flow,and molecular flow. The results show that the conductance values obtained by different simulation methods are in good agreement with the theoretical values in the appropriate flow regime range;the conductance formula for viscous flow under the blocking effect is no longer applicable;by adding the second-order slip boundary condition to modify the Navier-Stokes equation,the expansion of the conductance calculation in the transition flow region can be realized. Conductance calculations are performed in vacuum pipelines to reduce the computational complexity of the simulation. The research results provide a reference for method selection for conductance calculation of vacuum pipelines under more complex working conditions.