Abstract: There is a large difference in cathodic protection potential for the bottom plate of big-diameter oil tank, that will produce a poor-protection potential in the center plate of oil tank, while a over-protection in it will also produce when increasing the protection potential in the circumference direction. The maximum value of inhomogeneity of cathodic protection potential for bottom plate is calculated with superposition theorem of electric field and relative equations are derived. It is pointed out that the cathodic field is a major factor to influence the potential inhomogeneity of bottom plate of oil tank. The authors consider that double-anode or tetra-anode should be applied to protect oil tank.