Abstract: The top surfaces of foundations for vertical cylindral tanks in China are of cone shape, (i. e. high in center and low all around), the reason for that is to maintain this shape after foundation settlement and to be convenient for the drainage of deposited water. On the basis of the analysis on the actual shapes of the bottom plates after foundation settlement, the author holds that the designed cone shape will not be maintained after foundation settlement. During settlement, the area of bottom plates will become larger than that of tank foundation, and the circumferential tensile deformation of the outer bottom plates will be quite small due to the restrains from shell plates and edge plates. Under these circumstances, bottom plates deform unevenly, and the designed drains move off the lowest position. So the result is that the deposited water will be difficult to be drained, and the corrosion to bottom plates accelerated. This paper suggests that new tank foundation shape (i. e. low in center and high all around) shail be designed to prevent bottom plates from corrosion resulted from foundation deformation after settlement.