A multi-factor diffusion model of chloride in concrete was formulated, in which the hypothesis of Fick’s Law were corrected through parameter definition. The values of the parameters in the model were given for practical applications according to pilot calculation and analysis of mass data. By using this diffusion model, three calculation methods of concrete structure service life were further deduced, it includes definite value method, probability performance based method and time-dependent reliability based method. The service life prediction for the approach bridge of Huang-pu Bridge was carried on by using various prediction approaches mentioned above. The results showed that: time-dependent reliability based method can consider randomness of chloride diffusion coefficient and the depth of concrete covering layer, and the stochastic characteristic of environment variable can be easily taken into account too. By adjusting the target reliability, the prediction results are consistent with those of the other two methods.