As for the transmission channel in airborne laser communication, research onto combined transmission effects, including channel attenuation, intensity scintillation, beam-wandering and aero-optics effects in the atmospheric channel, were analyzed. Furthermore, transmission performances onto received beamwidth, link outage probability and bit-error rate were derived. The simulation results show that the received beamwidth is boosted by beam-wandering and aero-optics effects, and the beam size at low altitude condition is larger than the high altitude case due to severe atmospheric turbulence. On the other hand, long-range transmission links need to take into account the beam-wandering effect, which can be suppressed by optimizing transmitted laser beamwidth.