Abstract: The working principle of laser ranging/tracking unit in opto-electronic landing guidance was expounded. The laser emitting waveband was discussed and determined from the two main factors, eye-safe and atmosphere-window. The 1.5x m laser-s atmosphere transmittance was studied by comparing the results of empirical formula calculation and MODTRAN experiment. Based on the outcome of the precise calculation of atmosphere transmittance, the operating ranges of the laser unit under different meteorological conditions were analyzed. Results show the maximum operating ranges of the laser unit are 10.8 km and 8.5 km respectively in clear and haze conditions, which meets the requirement of opto-electronic landing guidance system for the tracking distance of more than 6 km.