Corrosion is among the most critical factors leading to the failure of reinforced concrete (RC) structures. Less work has been devoted to nondestructive tests (NDT) to detect the corrosion degree of steel bars. The corrosion degree was investigated in this paper using an NDT method based on self-magnetic flux leakage (SMFL). First, a mathematic model based on magnetic dipole model was settled to simulate the SMFL of a V-shaped defect caused by corrosion. A custom 3-axis scanning device equipped with a magnetometer was used to scan the SMFL field of the 40 corroded steel bars. Experimental data obtained by scanning the 40 steel bars showed that the B_Z curve of SMFL was consistent with the theoretical model analysis. Inspired by the qualitative analysis of the results, an index “K” based on a large number of experimental data was established to characterize the corrosion degree of steel bars. The experimental index “K” was linearly related to the corrosion degree α of steel bars. This paper provides a feasible approach for the corrosion degree NDT, which is not affected by the magnetization history and the initial magnetization state of steel bars.