Most of the previous studies on corrosion of the steel bar in reinforced concrete structures is based on the assumption that the bar is uniformly corroded. The actual measurement in projects proved that most of the reinforcement bar is in nonuniform corrosion state. Based on electrochemical principles of metal corrosion and combined with the real corrosion course of the bar in concrete structures, a theoretical model of the contour line of reinforcement bar due to nonuniform corrosion is established, through deducing the speed of corrosion and theoretical corrosion penetration and volume changes due to corrosion of reinforcement in general air at different position and different time. At the same time, the model is verified with the results from relative references and measured data.