Abstract: By the relative codes, the diameters of storage tanks with dome roofs (or internal floating roofs) together with ribs welded on cover plates are advised not to exceed 32m. The key of enlarging the storage tanks is to enlarge dome roofs, so the finite element method (FEM) is adopted in this paper to solve the above calculations in large span. For a 20 000 m3 storage tank, a model is established to meet the calculation precision. The strength and stability results under internal pressure and stability results under external pressure presented can be used as a theoretical basis for engineering design.