Coupling efficiency expression of single-mode fiber to single lens and lens array with the same single lens diameter is analyzed under Von Karman turbulence. The structure of small caliber lens array is designed. It is proved that an array with each receiving aperture which is 2 to 3 times of the spatial coherent radius is more suitable for a long distant free space optical communicaton system under strong turbulence and long communication distance, because the system has best coupling effect. It is proved theoretically and experimentally that the coupling efficiency of lens array spatial light is better than that of single lens of equal diameter within sufficient distance.