According to the design load of steam generator, fatigue crack propagation of dissimilar metal weld joint were considered for the steam generator nozzle safety end of third generation pressurized water reactor (PWR) nuclear power unit. Based on finite element analysis, crack propagation analysis was carried out on dissimilar metal welds at the safety end of the circumferential plane crack joints on the inner surface of ellipse with a depth of 20 mm and a length of 120 mm. The mechanical response of the structures with defects was calculated in transient state of each design, and meanwhile the crack propagation was calculated during the whole lifetime. The fracture safety margin at the end of life of the component was verified. The results showed that, the maximum expansion of dissimilar metal weld was 2.2 mm in the direction of crack depth at the end of life, and then the fracture safety margin met the design specifications. In consequence, it provided theoretical basis and technical support for the acceptance of service defects of the third generation PWR nuclear power units, and even provided strong theoretical, methodological and data support for the structural integrity assessment of nuclear power plants, which had important theoretical significance and engineering application value.