Reinforcement corrosion has been one of the major causes of degradation of reinforced concrete structures. Identifying the internal damage signals of corroded RC structures will provide a theoretical basis for structural health monitoring of existing buildings. This study investigated the effect of steel corrosion on the flexural characteristics of RC beams at the microscopic crack level using acoustic emission （AE） technique. The rise time/peak amplitude （R/A）， ringing count/duration （AF） and improved b （I_b） values of the acoustic emission signal were calculated for reflecting the changes in cracking patterns and damage modes of corroded RC beams. The results show that the ultimate load capacity of rusted reinforced concrete beams decreases significantly with the increase of reinforcement corrosion rate. there is a good correspondence between acoustic emission signal and concrete damage， and the damage process of the specimen can be divided into three damage stages： initial damage stage， damage evolution stage， and sustained damage growth stage. Besides， the trend of b-value curve can reflect the formation and development of cracks. Moreover，with the increase of load， the proportion of shear cracks in the specimen gradually increases， and the higher the degree of corrosion of the specimen in the sustained damage stage， the higher the proportion of shear cracks.