Abstract: The tensile and fracture properties at 20℃ room temperature and -158℃ low temperature of 9Ni steel welded joint, base metal and weld were obtained by experiments, and the effects of temperature on the properties were compared and analyzed. Approximate R6 option 2 failure assessment curves (FAC) for the base metal and weld, as well as FAC for the 9Ni steel welded joint were established and compared. The results show that the tensile and fracture properties of the base metal and the tensile properties of the joint and weld at low temperature were higher than those at room temperature, but the fracture toughness of the weld hardly changed with temperature. The yield strength and fracture toughness of the weld at different temperatures were lower than those of the base metal, and the weld was a weak area of the 9Ni steel welded joint. The envelope area of the approximate option 2 FAC constructed with room temperature tensile properties was slightly smaller than that with low temperature tensile properties, so the approximate option 2 FAC constructed with room temperature tensile properties can be used for safety evaluation of 9Ni steel welded joints. The FAC of 9Ni steel welded joint constructed by segment selection of the approximate option 2 FACs of the base metal and weld at room temperature can be used for safety evaluation of 9Ni steel defect-containing structures at low temperatures in the most conservative way.