To study the decline of RC structural resistance and durability caused by chloride ion erosion, based on RC beams accelerated corrosion test, firstly, the model of steel bar section area is analyzed considering the two kinds of uniform corrosion and pitting corrosion at the same time. Secondly, the relationship between the crack width and chloride diffusion coefficient and the resistance of RC beam is established. Then the structural resistance calculation model is discussed under the influence of corroded main bars with different corrosion degree in the same RC beam. Finally, considering spatial variability of material properties and structure size, the time-spatial-varying degradation model of RC beams resistance is built and the resistance of the bridge after 100 years of service is predicted. The results show that when the spatial variability of the parameters is considered, the resistance value wihtout considering the influence of the crack is 9.91% higher than the corresponding value considering the influence of crack. Considering the spatial variability of the parameters, the resistance of RC beam is decreased by 14.68% when compared with that without consideration. The appropriate choice of the fluctuation coefficient and the unit size to consider the spatial variability of the parameters of RC structure is very important for the safety evaluation of RC beam. The residual resistance of the structure in tidal area and splash zone is 43.23% and 36.45% of the initial value, compared with the atmospheric environment of the coastline, decreased by 6.73% and 21.35%, respectively. Therefore, in the service of the structure, corrosion protection measures should be done well to reduce the damage of the structure durability of chloride ion erosion.