Reinforced concrete structures generally work with cracks. Base on the studies at home and abroad, a crack influence coefficient about concrete carbonation γ_c, is defined. Mortar and concrete specimens with prefabricated cracks have been made for carbonation test. And the effect of water cement ratio, carbonation time, environment relative humidity, crack width, crack depth on γ_c is analyzed. A calculation model about carbonation depth in concrete cracks is obtained. Actual projects have been implemented to validate the model. It is shown that the model is applicable when the width of cracks in the range of 0.06~0.7 mm, and the bridge operation time have no significant effect on γ_c.