Based on the great difference of permeability between steel reinforcement and the media such as rust, water and concrete in reinforced concrete structure, magnetic based steel corrosion detection apparatus(MCD) was assembled, including embedded sensor, data collector and computer software system. Through calibration test, the relationship between the voltage change rate measured by MCD and the corrosion rate of steel reinforcement was characterized as a good linear relationship, and the corrosion of steel reinforcement can be quantitatively detected according to the linear fitting formula. The accelerated corrosion test of reinforced concrete beams embedded with sensor showed that the MCD steel corrosion apparatus had high measurement accuracy, and the time varying curve of the corrosion rate calculated can reflect the dynamic change of steel corrosion in concrete.