Chloride ion erosion in offshore environment may damage the mechanical properties of beam bridges. In this study, the reinforced concrete specimen was designed, accelerated erosion experiments were carried out to simulate the coastal corrosion environment, and the corrosion rate, nominal strength and equivalent strength of steel bars, concrete cracks and reliability of beam bridges were calculated to understand the time-varying mechanical properties of beam bridges. The results showed that the nominal and equivalent strength of reinforcing bars decreased with the increase of corrosion rate of reinforcing bars. The change of yield strength was greater than that of equivalent strength. The change of crack width of concrete showed a slow-fast-slow trend, and the reliability of beam bridges decreased significantly in about 50 years. The experimental results show that chloride ion corrosion can significantly damage the mechanical properties of the beam bridge and affect the time-varying reliability of the beam bridge. Therefore, it is necessary to carry out timely maintenance and inspection and take effective methods to control steel corrosion to ensure the safety of the use of the beam bridge.