To study the influence of the serious corrosion of I-shaped steel in the primary support of an undersea tunnel on the durability of the structure, relationship between corrosion rate and corrosion expansion force, and development process of corrosion expansion force under different concrete protective layer thicknesses are studied via laboratory tests. Thereafter, the simulation is used to study the concrete cracks, stress, strain, and cracking corrosion expansion force under different concrete protection layer thicknesses. The results show the following: (1) The corrosion expansion force increases with an increase in corrosion rate under the same protective layer thickness, and the corrosion expansion force increases with an increase in the thickness of the protective layer under the same corrosion rate. (2) The corrosion expansion cracking of the I-shaped concrete members is concentrated in the middle and two corner points of the flange. The concrete crack in the middle of the flange develops from the outside to the inside, whereas the concrete crack in the area of the two corner points develops from the inside to the outside. (3) The concrete crack stress obtained by numerical calculation is larger than that in the test.