Laser designation distance of electro-optical targeting system is an important design input of the laser designator and the fire control system.By using the energy transfer sequence of laser-guided system, we obtained the energy density at the entrance pupil of the seeker, and analyzed the factors that have effects on the designation distance.The empirical equations of target reflectance factor and atmospheric transmittance were deduced, and the functional relationship between the designation distance and such factors as laser energy, seeker sensitivity threshold, the divergence angle and the size of target, etc.Then the designation distance was obtained with the aid of Matlab. The calculation result showed that:1) The designation distance increases with the laser energy, and the slope decreases obviously when it achieves a certain distance;2) For a surface target, the maximum designation distance is reached when the target normal is parallel to target-seeker direction;and 3) There are some other important factors that have effects on the designation distance, including the attack height, the target size, the seeker sensitivity threshold, the atmospheric parameters, and so on.