Abstract: The strength reduction method is proposed to study the safety and stability of a submarine immersed tunnel made of reinforced concrete under static loading. Firstly, based on the strain compatibility principle considering the distribution and mechanical characteristics of reinforcing bars, formulas for calculating the shear strength and the tensile strength of the equivalent model of reinforced concrete structures are derived. Secondly, a non-linear numerical model for the strength reduction method considering structure-soil-seawater interaction is established by means of FLAC^3D software to simulate the failure of the immersed tunnel. The simulated results show that a sudden change of displacement can not be used as one of failure criterions for the immersed tunnel. The safety factor of the immersed tunnel is obtained considering the plastic state, shear strain increment and relationship between the tunnel roof displacements and the reduction factors. By using the strength reduction method, the failure mechanism of the immersed tunnel is revealed. Failure modes of the immersed tunnel section are mainly the shear failure and the tensile failure distributed near the center of the tunnel roof slab and the intersection between the roof slab and the two lateral walls.