In high speed free space optical communication systems, the efficiency that a laser beam is coupled into a single-mode fiber is an important factor. However, the laser propagation through atmospheric turbulence degrades the fiber coupling efficiency. According to the relation between turbulence intensity and optical system resolution, this paper explores the effect of atmospheric turbulence on fiber coupling efficiency, and derives the relationship of the receiving aperture, focal length, the wavelength of incident light, fiber radius and atmospheric coherence length with the single mode fiber coupling efficiency. Two methods were proposed to improve the fiber coupling efficiency in a turbulence environment. The first method is to maintain the coupling efficiency to be the maximum by adjusting the focal length of the coupling system when turbulence environment is deteriorated. The second one is to use a taper fiber in receiving end of the focal plane to improve the coupling efficiency. Finally, corresponding analysis, simulation and a coupling efficiency experiment at outdoor were performed, and it verifies the effectiveness of the improved method for fiber coupling efficiency.