By the wetdry cycle test on reinforced concrete specimens under natural conditions for two years, corrosion degree of rebars after concrete cover cracking and its influencing factors were measured. Based on the test results， applying the knowledge of mathematical statistics to analyze the corrosion characteristics of samples, a prediction model for the corrosion depth of reinforcement was established in relation to the concrete cover thickness, surface cracks width, bar diameter, concrete strength, stirrup spacing, and so on. The sensitivity analysis to model parameters shows that crack width on the surface is the most important factor for the corrosion depth of steel; stirrup spacing also has a significant effect on the corrosion depth of longitudinal reinforcement; the smaller the stirrup spacing is, the more remarkable the effect is. The experimental data show that the model is of good applicability.