Ladder shaped electrode monitoring system was designed based on the self potential method to study the corrosion processes of plain steel bars which were embedded at different depths in concrete. Results indicate that the embedded plain steel bars corrode in turn with the increasing of cover thickness. The interval between the initial corrosion times of two adjacent plain steel bars grows gradually. The relationship between the initial corrosion time and cover thickness can be fitted by exponential equation. The fitted results coincide with experimental data well. The critical chloride ion contents for plain steel bar corrosion are 03%~05% （by mass） in two types of concrete. The ladder shaped electrode monitoring system can trace the corrosion process of steel bar in concrete. In this way it provides early warning in time against potential safety loss of concrete structures.