In order to minimize optical distortion of beam introduced by gas jet stream and improve optical transmission quality,a project of utilizing N2/He mixture as working gas is proposed based on matching index-of-refraction and equalizing pressure. The method for determining gas mixture composition is also discussed. Experiments are carried on N2 and N2/He mixture respectively,and the laser beam wavefront aberration peak-to-valley (PV) value is measured from the data collected by Hartmann sensor. The results show that the wavefront distortion PV value of aerodynamic window (ADW) is 0.18 μm approximately in case of N2;while that is 0.13 μm in case of N2/He mixture. The optical quality of the ADW can be improved by using N2/He mixture as working gas instead of N2.