Abstract: The coupling efficiency of received optical power into a single-mode fiber is degraded by atmospheric turbulence in a free space optical communication system. The expression of average coupling efficiency of a collimated Gaussian beam into a single-mode fiber was derived under Kolmogorov turbulence channel, according to the theory of weak turbulence statistics. The relations between average coupling efficiency and turbulence intensity under different received aperture in 1 km propagation path were also simulated. The results show that coupling efficiency decreases below 0.1 when refractive-index structure constant rises to 10-12, and coupling efficiency degrades when the received aperture increases. A 37-element adaptive optics system with H-S wavefront sensor was adopted to compensate the efficiency. Compared with long exposure imaging and centroid position offset in different status of AO system, it was found that the P-V value and standard deviation decreased, Strehl ratio and coupling efficiency increased in low-order aberration correction. And things became better when high-order aberration was corrected.