The latest experimental study on steel reinforced high strength and high performance concrete (SRHSHPC) specimens shows that there exists interfacial bond softening phenomenon between embedded steel and high strength and high performance concrete (HSHPC), and it makes the shear transfer capacity between shaped steel and HSHPC be progressively reduced. To predict failure load in design, a theoretical model for interfacial bond softening behavior is required. As interfacial bond softening behavior is a nonlinear process involving material properties, it can be analyzed once the relation of interfacial bond stress (τ ) and slippage ( s ) is known. In this paper, the mechanism of interfacial bond-slip is studied, thus a simplified τ − s relation including ascending and descending parts is proposed and employed to analyze the interfacial nonlinear bond-slip process. Based on the interfacial equilibrium between steel and HSHPC as well as the τ − s relation, the basic governing equations in both softened region and elastic region are established and solved for steel strain or stress. At last, the application of the model is verified through comparison with experimental results. The calculating results of the model are found to be in good agreement with experimental results, showing that the model can describe the bond-slip process in real material systems.