To study the probability distribution characteristics of a single-mode fiber coupling's receiving efficiency fluctuations during atmospheric laser communication, a calculation model of transient coupling efficiency disturbed by atmospheric turbulence was established, and the least dimensionless parameters affecting the statistical distribution of the fiber coupling efficiency were obtained. The results show that as the ratio of aperture radius to atmospheric coherence length increases, coupling efficiency decreases, whereas when coupling geometry parameters increase, coupling efficiency increases initially before decreasing, indicating a single maximum point.The transient coupling efficiency of space light to single-mode fiber was simulated using the Monte Carlo method and analyzed using the distribution characteristics. The dual Johnson S_B distribution method was proposed to fit the probability density function of the transient coupling efficiency, and it was verified by the experimental data.The present results can provide references for optimized space-light parameter design in a single-mode fiber coupling system under different atmospheric turbulence intensities, supporting the realization of atmospheric laser communication systems with fiber coupling.