The bolted flanged joint with metal C-rings is commonly applied to long-term services for several decades. It has been affirmed that the produced stress relaxation can cause the sealing load to continuously decay during the long-time service, which is unfavorable for the sealing performance. This paper presents a finite element (FE) method to investigate the stress relaxation characteristics of the bolted flanged joint with a metal C-ring. The Nonlinear Isotropic Hardening constitutive model is used to describe the rate independent plasticity of all the materials, and the Strain Hardening equation is used as the creep constitutive equation. In the FE model, the bolted flanged joint is considered as a whole, and the stress relaxations of all the components including of the gaskets, the bolts and the flanges are calculated by the unitized method. The method can be adopted to investigate the stress relaxation characteristic of the bolted flanged joint at any time, and estimate the active time when the critical sealing load has been defined. By comparison with the performed experiments, the FE method was verified. Based on the proposed FE model, some simulation results are presented and discussed, which mainly correspond with the relations of some design parameters of the metal C-ring with the stress relaxation of the bolted flanged joint. It explains how the stress relaxation characteristic is influenced by these parameters, which is helpful for the design, assemblage, and maintenance of the bolted flanged joint with long-term service requirements.