The composite rapid soil stabilizer (CRSS) developed by the research group has the characteristics of rapid setting and high strength. The CRSS is used to rapidly solidify the surface sludge as the overlying hard shell layer. Through model tests, the failure form of the hard shell layer, p-s curve characteristics, deflection basin shape and earth pressure distribution under different overlying hard shell layer thicknesses (3cm, 6cm, 9cm and 12cm) are investigated. The test results show that with the increase of the thickness of the hard shell, the failure mode of the hard shell layer changes from folding failure to punching failure. The greater the thickness of hard shell, the more obvious the diffusion effect, the higher the ultimate bearing capacity and the corresponding increase of settlement displacement. The characteristics of the central settlement displacement of the bearing plate can be described by the segment slope of the p-s curve. The settlement displacement at different distances from the center point of the load plate can be described by the shape of the deflection basin. A modified logarithmic assumption deflection basin deformation formula is proposed, which is more reasonable than the linear assumption deflection basin deformation formula. The earth pressure decreases from the center to the outside, and decreases gradually along the depth direction. With the increase of the thickness of the hard shell, the earth pressure distribution is more uniform. The research results provide a theoretical basis for the engineering application of CRSS.