Abstract: Using the method of the eignvalue and eignvector analysis of chemical system to study the chemical lifetime of sodium layer. The results show that the chemical lifetime of sodium layer calculated in this paper is very different with the traditionally defined chemical lifetime. The results indicate that in the region near the peak of sodium layer, the dynamical transportation predominant than the photochemical process. But in the region of the bottom side of sodium layer, the photochemistry is the main process. The Na layer can be regarded as a tracer of dynamical perturbation. The results of this paper provide a theoretical basis for analyzing the Lidar observations of sodium layer.