A method of steel bar corrosion monitoring used single mode-multimode-single mode optical fiber was proposed. Firstly， three lengths of non-core fiber （NCF） were used as multimode fiber to make SMS optical fiber sensor， including 2 cm， 3 cm and 4 cm. Then， the fiber was directly placed on the polished side surface of a steel bar， and immersed in 3.5% NaCl solution for corrosion test. The change of light spectra was continuously monitored with an optical spectral analyzer （OSA）， and the corrosion potential and corrosion current density of steel bar was measured with open circuit potential and linear polarization resistance， respectively， based on which the accumulative corrosion-induced mass loss of steel bar was calculated. Results indicate that a cubic relationship is present between the mass loss of steel bar and the change in the characteristic wavelength of SMS optical fiber. The sensitivity of SMS optical fiber for corrosion monitoring increases with an increase of the length of the multimode fiber segment， while it first increases and then decreases with the increase of corrosion time.