Laser illumination is an effective technology for the imaging and tracking to the far dim objects in the air. The laser intensity spatial distribution on the target plane fluctuates usually remarkably, which will decrease image resolution and tracking accuracy in optical system. In this paper the parameters for evaluating illumination effect have been discussed, and power spectral density (PSD) of the illumination laser intensity fluctuation on the target plane is adopted to evaluate the intensity uniformity. A model of illumination laser propagating in atmosphere has been built, numeric simulation for laser illumination to air objects has been carried out and the illumination intensity fluctuant power spectral density and object image have been obtained. Being contrasted with experiment result shows that the simulation is reasonable.