To solve the damage problem of low slow small target caused by laser irradiation, a model of low slow small target (LLST) irradiated by a 1.06 μm slant path transmission laser is established. Through simulation calculation using the model, the paper presents the transmission rate vs. the zenith angle and the altitude in various atmospheric visibilities and the normalized power density vs. time in different zenith angle. The factors of the normalized power density are analyzed, such as the atmospheric visibility, the zenith angle and the altitude. Furthermore, the best condition that the zenith angle can be determined in the process of the laser defense low slow small moving target, the surface energy is the largest in the irradiation time.