In order to study the effect of the air pressure on the coupling coefficients for laser propulsion, a vacuum chamber was designed in which the air pressure can be changed from 4.6×103 Pa to 101.3×103 Pa. The pendulum with light-craft model was propelled by TEA CO2 laser. It was found in the experiment that when the laser pulse energy was 20 J, the coupling coefficients increased gradually with the increase of air pressure, reached the summit at the air pressure of 34.6 kPa, then began to decrease. When the laser pulse energy was reduced to the 10 J, the air pressure value at which the coupling coefficients reached the summit fell. According to the theory of shockwave and laser plasma, the paper gives a preliminary analysis to this phenomenon and predicted the effect of the laser pulse energy on the peak value of coupling coefficients.