In order to solve the problem of erosion and durability of reinforced concrete bridges in saline alkali environment in Northwest China, through field investigation of bridge diseases in saline alkali environment in Northwest China and the distribution of chloride ion in soil and bridges, the chloride ion diffusion law of each part of bridge was analyzed by modified Fick’s second law and finite element simulation, and the durability life of a proposed bridge was predicted. The improvement effects of measures such as improving concrete strength and protective layer thickness on the durability life of bridge were discussed. The results show that the chloride ion content in the surface soil is the highest in saline alkali environment. There are many diseases in the service bridge, and the substructure is the most vulnerable to chloride ion erosion. The durability life of substructure bearing platform, pier and pile foundation of service bridge is 55.3, 63.1 and 91.5 a respectively. The durability of bridge can meet the requirements by reasonably improving concrete strength and protective layer thickness.