Based on magnetic medium theory, a newly invited magnetic corrosion detector (MCD) showed great potential in quantitative monitoring the corrosion process of steel rebar in concrete. To furtherly explore the application performance of MCD, a numerical model was established to simulate the magnetic induction intensity variation caused by corrosion of steel rebar. And the influence of the length and the positioning variance of steel rebar on the measurement were discussed based on experimental and numerical investigation. The research results indicate that the numerical simulation is a valid approach to reveal the essential performance of MCD in measuring the corrosion ratio of steel rebar in concrete. Based on the geometric and physical properties of MCD, the corrosion ratio of the steel rebar can be effectively measured. And the positioning variance of steel rebar has significant influence on the precision and sensitivity of MCD. It is suggested that the steel rebar should be placed right at the center of the test area during the measurement to achieve a high accuracy.