Water and sand gushing will lead to the destruction of upper cohesive soil that further form cavity, and the ground collapse caused by the development of soil cavity adversely affects the geotechnical building or foundation. In this study, the theory of elastic mechanics clamped beam was used to establish the arbitrary point stress calculation model and the stress solution of the clay soil under the action of gravity was derived. According to the equilibrium state of ultimate stress and unified strength theory, the formula for calculating the critical radius of cavity induced by water and sand gushing was obtained. The parametric study of above formulas shows that the cavity radius decreases with the increased depth of clay layer and Poisson’s number of clay layer. The higher the clay cohesion is, the higher is the cavity clay. Moreover, the limit radius of cavity first increases and then decreases with the increased of clay thickness. The results of theoretical analysis of critical collapse radius and the results of model test demonstrate that the calculating value of critical collapse radius based on unified strength theory is consistent with the test results.