Abstract: In order to facilitate the designers of welded steel tanks to calculate the seismic performance of tank wall through choosing appropriate design codes, this paper presents the calculation methods of maximum axial stress and allowable critical stress of bottom shell under GB 50341-2003 and API 650-2013. Taking the 15×10⁴ m³ and 10×10⁴ m³ dual-disc floating roof tanks as an examples, the maximum axial stress and allowable critical stress of bottom shell are respectively calculated. The results show that the maximum axial stress calculated by the method mentioned in GB 50341-2003 is the same as that of API 650-2013; both GB 50341-2003 and API 650-2013 are applicable for allowable critical stresses, but GB 50341-2003 is conservative in the calculation. The formulas for allowable critical stresses in GB 50341-2003 and API 650-2013 can be replaced by one formula. When the maximum axial stress is not greater than the allowable critical stress, the bottom shell of tank is safe under the earthquake.