Abstract: A case analysis on the perforation of bottom plate of a 30 000 m³ floating roof tank is made. It is pointed out that the impact of roof columns to the tank bottom plate when liquid level is up and down and chemical corrosion are the major causes to the perforation of bottom plate. The paper introduces the maintenance program and repairing process. The article considers that the floating deck should not be settled down to the bottom plate if possible (except special needs) to avoid the impacting in daily operation of tank, the lower sulfur content for the crude oil stored is, the better it is, and the corrosion control measurements should be taken into account prior to the construction of oil tank.