Based on the Gamma-Gamma distribution of the intensity fluctuation for light propagating in random media, the statistical properties of received photons of a lunar laser ranging (LLR) system are studied, in which an adaptive optics system is equipped to correct the effect of atmospheric turbulence. A completely statistical description, i.e., the probability density functions of the fluctuation of the received photons, is presented which takes into account most of optical processes. On this basis, a series of characteristic quantities of the received photons, including average value, fluctuation variance, cumulative probability function, and so on, are calculated in the general conditions of the atmospheric turbulence. With these characteristic quantities, a performance evaluation of the LLR adaptive-optics system is obtained quantitatively.