The spatial diversity can be used to mitigate atmospheric turbulence over airborne optical links.Based on the direct detection receivers,the outage probability and power gain of optimal combining (OC) spatial diversity are compared with those of equal gain combining (EGC) and select-max combining (SMC) diversity.The power gain of these diversity systems is found to be substantial,and the performance of optimal combining is better than those of equal combining and select-max combining,as the performance of equal combing is found to be almost as good as that of optimal combining.But optimal combining is more difficult to implement.Hence the diversity technology should be chosed after comprehensive consideration in actual airborne laser communication engineering applications.