According to the mechanism of “ripping up the Riverbed” and lifting mode of clay block, constructed a lifting analysis model of clay block broken. Considering the combined effect factors of clay block lifted (for example, clay block density, thickness, mechanical strength, peak flow and sediment concentration, the duration of the flood process, river channel morphology parameters, fluctuations lifting force in the velocity indirection of siltation layer etc. ) and the most adverse load combinations conditions, making use of the flexural strength experimental data of clay block on “Ripping up the bottom” river reach, establish critical lifting mechanics equation of clay block and solve it. The research results enrich calculation theory of "Ripping up the Riverbed”.