Atmospheric turbulence and diffraction could cause the laser beam to diverge， which could affect the performance of laser propulsion. In laser propulsion， the size of the laser spot must cover the diameter of the lightship. The size of the laser spot reaching the lightship was used as the standard to analyze the influence of atmospheric turbulence and diffraction on the beam divergence during CO₂ laser propulsion. The analysis showed that in the case of weak turbulence， the laser zoom launch system ensured that the beam could cover the surface of the lightship during laser propulsion. For laser beams with different beam quality factors， the minimum aperture of the launching system and the relationship between the focal length of the launching system and the distance of the lightship were obtained.