The widespread distribution of riprap in estuarine mudflats has brought significant challenges to the penetration construction of steel casings. To reveal the effects of casing length, diameter and wall thickness on the stress and deformation, as well as the deformation characteristics and mechanical behaviors of the steel casing during the sinking process, the paper utilizes finite element method to construct a three-dimensional numerical model of the collision between steel casings and riprap in mudflat. The research results indicate that longer steel casing has better crushing effect on the riprap, smoother deflection curve of the casing body and smaller deformation at the casing end under the same casing diameter and wall thickness conditions. Under the same casing length and wall thickness conditions, the steel casing with a larger diameter has a better crushing effect on the riprap and smaller deformation at the casing end. As the casing diameter increases, the stress values of S11 and S33 in the soil at the casing end gradually decrease, and the range of stress concentration gradually increases. This study can provide a theoretical basis for the design and construction of steel casings in the riprap environment of the mudflat near the estuaries.