The experiment of coupling coefficients for TEA CO_2 laser propulsion (air mode) was reported at various pressures.It is found that the coupling coefficient reaches the maximum value at the pressure of 34.6 kPa,which was similar with the case of the experiment,in which TEA CO_2 laser pulses were focused on the surface of solid targets at different air pressures.In that experiment,the maximum coupling coefficient was also measured at the air pressure of 34.6 kPa.According to the theory of shockwave and plasma at high temperature,the paper gave a elementary analysis and explanation to this phenomenon.