C-ring, whose sealing behavior is the key to the sealing structure, is one of the main seal-rings for Reactor Pressure Vessel. A finite element model with less simplification has been established using ANSYS APDL to simulate the sealing behavior of C-ring. The numerical results, including sealing behavior curve, specific pressure and springback value, have been compared with test results at 7.67%, 8.42% and 8.09% compression ratios respectively. It confirms the validity of this numerical simulation method. With the numerical method, the deformation and stress distribution and sealing behavior curve of C-ring can be obtained. The behavior curve can be divided into 5 sections. At 10% compression ratio, the peak contact stress is 501MPa, the contact width is 3.22mm, the total springback value is 0.460mm, the effective springback value is 0.332mm and the linear load is 788N/mm. As C-ring is composed of three parts, three models, i.e. spring-only, spring with inner lining and spring with two linings, have been simulated to study the effect of the contact states between each parts on sealing behavior.