According to the analysis of the current chloride ion eroding model, it has been known that diffusion coefficient and chloride ion concentration on the concrete’s surface are random variables. Meanwhile the concrete protective thickness is also random variable because of manufacturing error and other reasons. So the diffusion process is the random process in fact. The random calculation model of chloride ion concentration in concrete has been deduced from the current model with the help of mathematical knowledge in this paper. Moreover, a satisfied effect has been achieved after comparing the calculated values with the test values. The calculation model is proved to be useful for life prediction and assessment of concrete structures.