A mathematical model was established for the cathodic protection potential distribution outside of tank bottom, in which the inhomogeneity of medium was taken into account. The effect of soil resistivity change along vertical direction on potential distribution was studied through numerical simulation. The simulated results indicate that the non-uniformity is further intensified due to the bitumen sand base; soil resistivity in the position where auxiliary anodes are placed has significant effect on potential distribution; there is a buried depth of anode for optimal potential distribution under different geological conditions.