Abstract: The paper is concerned in the calculation and assessment of the stress intensity for main vessel in China experiment fast reactor(CEFR), and introduces stressly the new problems and methods related to the design, such as the assessments of class1 conponents and hightemperature vessel, the calculations of the equivalent thermal conductivity, the temperature field and thermal stress, thermal shock, the cannula's load stress, and the gridding transition of the complex structure etc. The calculated results show that the stress intensity of vessel in CEFR meets the requirements of ASME code.