The invention discloses a C-shaped ring. The C-shaped ring comprises a hollow and annular cladding layer, a middle layer and an internally-installed layer, wherein the middle layer is contained in a hollow structure of the cladding layer and tightly clings to the inner wall of the cladding layer; the internally-installed layer is arranged in the hollow structure of the cladding layer and clings to the inner wall of the middle layer; the sections of the cladding layer and the middle layer are in a C-shaped structure; the internally-installed layer is an annular spiral tightening spring wound by a normal-quadrangular-shaped wire and front and rear ends of the spiral tightening spring are welded; a smooth cladding surface is formed on the outer surface of the spiral tightening spring; the cladding surface tightly clings to the inner wall of the middle layer. The C-shaped ring has a high sealing performance; when a medium pressure or/and a temperature is/are changed, the stability of a sealing performance is kept by a compression-rebounding property of the spiral tightening spring and the sealing performance of a reactor pressure vessel is guaranteed; the C-shaped ring can be adaptive to high temperatures, high pressures, instant large temperature difference, and instant pressure fluctuation working conditions of a nuclear power station; meanwhile, the machining difficulty is greatly reduced.