As for the transmission channel in airborne laser communication, research onto combined transmission effects, including channel attenuation, intensity fluctuation, beam-wandering and aero-optics effects, are analyzed. Furthermore, transmission performances onto received beamwidth, link outage probability and bit-error rate are expressed in closed form. Simulation results show that the received beamwidth is boosted by beam-wandering and aero-optics effects, within which the value of the low altitude condition is larger than the high altitude case due to severe atmospheric effects in the low altitude case. On the other hand, the beam-wandering effect for the long-range transmission link should also be considered, and could be reduced by optimizing transmitted laser beamwidth at the transmitter end.