Abstract: An investigation on the seal of inner floating deck of oil tank revealed that failures in static seal and dynamic seal often occurred on their post. The former included the gap or hole appearance at overlapping parts of cover plate of inner floating deck, gap appearance at compact parts between the edge of inner floating deck and seal capsule or tongue-shaped seal, seal failure between anti-rotation duct and cover plate of inner floating deck and seal failure among automatic air vent, manhole and cover plate. The latter included the poor performance of the seal among seal capsule, tongue-shaped seal and inner shell of tank, poor performance of the seal between measuring duct and its protecting barrel cover, poor performance of the seal between manhole cover and base and tightness of anti-rotation steel rope not being in accord with the demand of safety operation. Based on causes analysis of seal failure of inner floating deck in design, construction, operation and maintenance, prevention measures are given about optimum design, quality control of construction, manufacture and installation of inner floating deck, regular detection and management of inner floating deck and standardized maintenance operation.