It is greatly significant to weaken the thermal blooming of beam path indoor by keeping the tube filled with single specific gas of uniform density and low absorption. The gas curtain is applied as sealing device, and the structure of gas curtain is elaborately designed. The fluid characteristic of gas curtain with nitrogen-filled inlet and air-filled outlet is modeled and simulated by the species transport model which is provided by the computational fluid dynamic software FLUENT. The simulated results indicate that the planar jet flow can indeed block off the refluence of the outside air. The gas density is undulant at the region with mixed nitrogen and air. However, the gradient direction of the gas density variation is parallel to the direction of beam transmission. This type of density variation only affects the piston item of optical phase, whereas it has no influence to the beam propagation. Finally, the experimental platform is established. The experimental results indicate that the designed gas curtain can seal the beam path indoor and reduce optical path difference.