Based on the principle of the fiber Bragg grating strain sensor as well as the volume expansion of the reinforcing steel due to corrosion,an optical fiber grating sensor for monitoring corrosion of reinforcing steel and the method of temperature compensation were studied in the present paper. The sensor construction is that one Bragg grating is stuck on the inner center of two bars against each other,and the reinforcement volume as well as the diameter will expand due to corrosion. Based upon sensing mechanism,monitoring will be carried out by transforming the diameter increase to the fiber strain, and as a result the degree and rate of reinforcement corrosion can be obtained. The principle of corrosion monitoring is that the strain induced by corrosion and temperature fluctuation is measured by a reinforcing steel fiber grating sensor. At the same time,the strain induced by temperature fluctuation is also measured by an individual stainless fiber grating sensor. Therefore by two independent fiber grating sensors,the volume changed by corrosion can be separated. By the concrete encapsulating and embedding method of FBG corrosion sensor,the degree of corrosion of reinforcing reinforcement will be measured directly,which is not affected by corrosion factors and can be used in the early corrosion monitoring of reinforcement in concrete structures. Finally the relationship between corrosion rate and shift in center wavelength was calibrated by experiment.