Abstract: Three methods are ayailable for the calculation of lower nodal point of large oil tank, they are the method of rigid foundation beam by APKJ. B. Denham), the method by the Mechanics Department of Chinese Academy of Sciences, and the method by the authors of the article, which couples elastic foundation beam with rigid foundation beam. Analysis made on the three methods indicatesthat the authors' method is suitable for the cases that annular plates have adequate length and the annular and intermediate plates are butt welded. The results obtained by using this method and that actually measured en the 100 000 m³ floating roof tank in Qinhuangdao are compared, and a due conformation is real zed between the theoretical and actual measurement. In contrast, consi derable errors are found in the other two methods. The paper concludes that (1) the width of the annular plates extending beyond shell plates shall be dully increased so as to minimize the bending stress on annular plates: (2) the annular and intermediate plates shall be butt welded to reduce the maximum stress of annu ar plates; and (3) tank wall deformation shall be considered in the calculation method for lower nodal point.