Abstract: A beam-gas-interaction model was developed by combining the scalar diffraction theory with the N-S equations. With this model, the high power density beam propagating through a Zshape tube with blowing non-absorbing gas in at different velocities was calculated and the emitting beam qualities at far field were analyzed. The results show that, the thermal effects on beam propagation will be efficiently restrained and the beam quality at receiving section at far field could be improved. Especially, the blowing process itself may bring an other greater influence which caused place of peak intensity and centroid of intensity distribution to be deviated from the ideal ones, and the higher the blowing velocity is, the greater the deviations are, the bigger the spead of beam spot is. So the ado