In order to reduce errors cause by Langmuir flow in ring laser gyros, controlling effect of bypass tube between cathode and anode on the Langmuir flow in discharge tube is investigated. Langmuir flow patterns in discharge tube as a function of bypass tube radius, discharge current and gas pressure are simulated numerically with gas glow discharge theory. It is found that Langmuir flow pattern varies with the said parameters. What′s more, average Langmuir flow across the fundamental mode section in discharge tube can be adjusted to zero. These foundings not only explain some experimental phenomena but also are useful in reducing Langmuir flow drift and consequently thermal sensitivity in ring laser gyros.