Abstract: Decay of sound field in an enclosure is regarded as a random process of sound particles. Different average parameters （the number of collisions n and the time t） and average methods （dB average and energy average） are discussed. The decay rate caculated by dB average method is always faster than by energy method. A statistical analysis model of sound decay is given. After discussion about initial state, stationary and ergodicity of sound particles, the mean free path and mean collision frequency are caculated. The relation l=c/v is not tenable. The collision probability and distribution probability are given on the supposition that collision process of sound particles is independent. Oppositely, considering it is not independent, the transfer probability is given as P_ij=S_j/(S-S_j,i‡j). The statistical analysis of decay of sound field in an enclosure is made, and different for mulae of reverberation time on different suppositions are deduced. The presuppositions of classic reverberation formulae r troubles are mentioned. Considering the transfer probability given in this paper, some properties of the model in which the sound decay process is regarded as a homogeneous Markoff process are discussed, and a re-verberation formula is given as T=0.161V/(-Slnβ),β, here β is the root of equation 1-\sum\limits_i = 1^m X_i/(X_i+β)=0, X_i=S_i(1-α_i)/(S-S_i).