Abstract: Galvanic protection distance (PD) and the effect of the addition of various contents of Cl^- on it were studied with galvanized auto steel sheet specimens whose zinc coating was partly removed and covered with Ying-tan soil mud. The results show that in the mud without Cl^- the PD value increase linearly with the thickness of mud layer, and the red-rust emerging time increases correspondingly. With increasing Cl^- content and thickness of covered mud, the PD value raises and reaches a maximum. Cl^- content influences not only the average corrosion current density, but also the dissolved oxygen, pH value and electrical conductivity. The thickness of mud layer and Cl^- content control the PD value and surface corrosion state.