Abstract: The actual corrosion status is analyzed of the rim plate of the No. 8 floating roof tank of 20, 000 m² in Cangzhou Initial Station. The main reason for the corrosion is the corrosive envirenment. This environment is caused by the water which has entered and failed to volatilize out of the narrow clearance between the rim plate and tank foundation. The drawbacks of conventional water proof measures closed type are discussed, and the open water proof structure introduced. According to optimum seeking, it is pointed out to cut away the exposed edge of circular beam and weld a steel ring onto the down side of rim plate. Through comparing this structure with cathodic protection, its conciuded that the former is characterised by quick and simple construotion, high adaptability, and good performance, while the later by high cost and poor performance for the No. 8 tank.