The calculation method of thermal stress of hull structure is summarized, and the temperature field of a handysize tanker at loading temperature of 80 ℃ is studied based on the thermal balance theory. The allowable cargo density based on interior bottom plate design is calculated. The yield strength and buckling strength of strong frame are calculated. The calculation results show that, after the Harmonized Common Structural Rules (HCSR) loads are superimposed, the thermal stress caused by temperature field can cause the bilge structural deformation, and a large increase in total stress can cause the structural yield and buckling failure. Therefore, even if the temperature is less than 80 ℃, the influence of temperature field should be properly considered in the design. The research method is simple and quick, and is of important engineering significance.