In order to have an accurate measure about modulation transmission rate of UAV′s optical communication tasks to ground in the current conditions, a channel model which is suitable for airborne atmospheric optical communication needs to be established. Firstly, the airspace range was divised, the influencing factors was limited in the troposphere. Then the comprehensive influence model was established based on tropospheric typical atmospheric effect, and further the channel transmission model was established under UAV′s optical communication to ground. Parameters setting about UAV was made under the existing conditions, and total amount of effective information was simulated under UAV′s optical communication to ground on different heights, the best height for carrying out optical communication tasks was obtained which was about 2 600 m. At this altitude, the channel transmission model was simulated by selecting appropriate modulation rate points. Combined with the critical effective total information′s setting and the simulation results of channel model, on the basis that the needs for feedback test about optical communication system was met, the critical modulation rate needs of UAV′s optical communication was locked in 1.12 Gbps. The result provides the evaluation standard for the design of the optical modulation system under existing conditions and it has a strong guiding significance.