Abstract: In order to understand the operation characteristics of vessel traffic flow more deeply, taking the study of speed dispersion as the entry point, this paper proposes two kinds of indexes to describe the speed dispersion characteristics of vessel traffic flow. According to the theory of traffic flow fundamental diagram, this paper quantitatively analyzes the distribution of speed dispersion on the fundamental diagram of vessel traffic flow and its relationship with the three parameters of vessel traffic flow. The results show that the distribution of speed dispersion tends to be stable with the increase of ship flow, density and velocity. The speed dispersion gradually decreases with the increase of ship flow in the low and high speed dispersion range, and increases with the increase of ship flow in the low speed dispersion range. The improved K-means and FCM（Fuzzy C-means） algorithm are applied to do the cluster analysis for VTF condition. In the end, four substates of VTF condition are proposed: stochastic state, equilibrium state, tradition state and nonequilibrium state. This research plays a role in better understanding the characteristics of vessel traffic flow.