The performance of mixed radio frequency (RF)/free space optical (FSO) airborne communication system based on decoding and 2×2 relaying is analyzed. The model of 2×2 relay-assisted mixed RF/FSO airborne communication system is established, and the probability distribution function and cumulative distribution function about signal-to-noise ratio of this system are derived by Meijer’s G function. Moreover, the closed expressions of average bit-error-rate (BER) and outage probability of the system are obtained. The effects of atmosphere turbulence intensity, aperture size and modulation mode on the average BER and outage probability are analyzed by the simulation. The results show that the aperture averaging effect can effectively improve the performance of mixed RF/FSO airborne communication system, and the performance of the 2×2 relay-assisted communication system is significantly better than that of the 1×1 relay-assisted communication system.