The strength and deformation behaviours of overconsolidated clay are influenced by various factors such as the initial void ratio, stress path, and preconsolidated stress. To describe the state dependency of overconsolidated clay, this paper introduces the density state parameter and stress state parameter, which help characterize the differences in void ratios between current and dyadic state points. These parameters are utilized in a new constitutive model for overconsolidated soil, which also includes a new state-dependent hardening rule to describe the evolution of the subloading surface where the current state point is located. The proposed model features a single-yield surface to simplify the formula, and its material parameters can be calibrated using conventional laboratory tests. The model's predictions are compared with triaxial test data of various types of clays under drained and undrained conditions, showing good agreement. The comparison results indicate that the evolution of the stress-state parameter and density-state parameter is influenced by the preconsolidated pressure, which reduces to zero once the clay sample reaches the final critical state. Additionally, in preconsolidated clay, the stress-strain curve of the soil sample demonstrates noticeable softening characteristics during the drained shear process, initially resulting in a volume decrease followed by an increase. In the undrained shear process, negative pore water pressure is generated.