Abstract: The finite element–immersed interface method (FE-IIM) is a numerical method for fluid–structure interaction (FSI) that combines finite element discretization with the interface jump treatment of IIM. It aims to achieve accurate resolution of physical quantity variations at the structure interface, but its practical application has been limited by poor numerical stability. In this study, a stabilized L2 projection-based FE-IIM coupling method is proposed. This method requires the structure interface to satisfy the C0 continuity condition and introduces finite element shape functions on the boundary elements, yielding continuous and controllable representations of interfacial physical quantities within the basis function space. Through the stabilized L2 projection strategy, the jump conditions of IIM across the C0-continuous interface are reconstructed, and an interpolation scheme is developed to project flow field variables onto the interfacial basis function space, effectively suppressing non-physical oscillations that tend to arise at high Reynolds numbers. In treating interfacial discontinuities, first-order derivative jump conditions are introduced into the interfacial basis function space, enhancing the computational accuracy of interfacial variables. Numerical tests with Reynolds numbers ranging from 200 to 50,000 demonstrate that the proposed method can accurately capture the major flow structures and resolve fine-scale variations of physical quantities at the interface. The stabilized projection-based FE-IIM coupling approach provides a promising theoretical framework for simulating FSI problems that require high-fidelity resolution of interface quantities such as pressure.