Abstract: Density is the most fundamental parameters in the calculation of debris flow movement characteristics. In order to explore the dynamic evolution of the debris flow density under different spatial-temporal conditions, based on the observation data of debris flow in the Jiangjia Gully since 1995 and the investigation data of debris flow hazards occurred in the Wenchuan earthquake area since 2008, the interaction of fluid-soil coupling process and density of typical debris flow disasters is studied, and several methods of density calculation are used in the Wenchuan earthquake area and the differences in the calculation formula establishment and the disaster characteristics are analyzed. The results show that the density changes dynamically with the evolution of formation conditions of debris flow. On the temporal scale, the activity of the solid materials in the gully is positively related to the density, and the density will decrease by 10.0%~27.8% when the solid materials evolve to highly active from extremely active. There is a power function relationship between the density and peak flow of debris flow, and it has an obvious limit when the density increases with the increasing flow. On the spatial scale, the change trend of density is consistent with the channel gradient, which increases with erosion and decreases with sedimentation. The deviation of these calculation results in the Longmen mountain area is in the range of 10~30%, suggesting that using only one method for density calculation is very much likely imprecise. Empirical formula are established based on the total amount of erosion and deposition, and particle parameters are practically applied to the debris flow at the Yongjia Gully. The research lays a foundation for calculation of the density based on the disaster history and evolution of geological conditions, and are of practical guidance significance in the evaluation and prevention of debris disaster and the calculation of dynamic parameters.