Abstract: This paper analyzes why oil is stored on the pontoon of floating roof of tank, and shows the reason relationship diagram for oil stored on pontoon from the example that oil has been stored on the pontoon of 10^# tank two times in Linyi pump station. According to the reason relationship diagram, it is clear that the deformation of side-wall made the gap between the side-wall too narrow and attachment of floating roof contacted side-wall directly, and the foam seal elastic element was pressed tightly, and the floating roof can not move up and down freely. Because the flange bolts of manway of pontoon had not been tighten, the cover of manway had been out of seal, the more the rain water stored on the pontoon, the bigger the load on pontoon, and the load was not even, so floating roof was sloped and blocked, the oil level raised over manway, and oil was slopped from manway, then oil was stored on the pontoon. This paper also shows some measures to protect oil stored on pontoon.