The strength and deformation characteristics of overconsolidated clay depend on state factors such as initial void ratio, stress path and preconsolidation pressure. To characterize the state dependence of overconsolidated clay, we define two state parameters: the density-state parameter and the stress-state parameter. The former describes the void ratio difference between the current state point and the corresponding state on the critical state line, and the latter describes that between the dyadic state point and the critical state line. A new stress-state equation is then derived. Finally, the state hardening rule is adopted to describe the hardening law of the subloading surface where the current state point lies, and a state hardening model suitable for overconsolidated clay is established. Comparisons between model predictions and triaxial compression test results of different clays under drained and undrained conditions show that the proposed model exhibits good performance. The results show that stress history affects the stress-state equation of clay. The density-state and stress-state parameters in this equation approach zero when the soil reaches the critical state, and they characterize the compactness and moisture state of the soil respectively. The proposed state hardening rule can reasonably describe the strain softening and dilatancy of overconsolidated clay under drained shear, as well as the effective stress paths and the evolution of excess pore water pressure under undrained shear. The established clay state hardening model has a simple form with a single yield surface. Its material parameters can be calibrated by conventional laboratory tests, so it has good prospects for theoretical application in engineering practice.