Abstract: In the light of the top unstability of oil tank, an analysis on the structures of spherical shell and stiffener is carried out from the reason influencing the stability of tank top and calculation. The article summaries the analysis methods and analysis steps of top unstability of oil tank and gives some equations to compute the top outer loading of tank and the top stability of tank. Some typical cases of top unstability of oil tank are studied. It is pointed out that the outer overloading of tank top, the thinner of top-plate by corrosion and defects produced in manufacture and installation result in the top unstability of tank. To avoid the top unstability of tank, the serious attention must be given to the design, manufacture, operation and maintenance of tank top.