The riverbed condition of “Ripping up the riverbed” is existence of clay block in the stratification siltation structure riverbed. The simulation test of “Ripping up the riverbed” validated that the mechanical properties of clay block being an important affecting factor. But the particle size distribution of clay block that formed through flocculation deposition is a key indicator of affecting clay block mechanical properties. Making use of particle size analysis test and shear strength test data of clay block of “Ripping up the riverbed” reach in the Yellow River, and using BP neural network to establish the relationship model between the clay block particle size distribution and its shear strength and solving it. The results validated the model is correct and has good generalization ability. The research result provide a studying method for analyzing the relationship between the particle size distribution of clay block and its mechanical properties and also provides condition for enriching research theory of "Ripping up the riverbed".