Abstract: In order to evaluate the structural integrity of dissimilar metal welded joint accurately, the weakest position of the fusion zone of 52M Ni-based alloy dissimilar metal welded joint used in the safe end of nuclear power plants was selected, and the fracture behavior of dissimilar metal welded joint under the interaction of in-plane and out-of-plane constraints was studied by fracture mechanics test at room temperature and fracture morphology observation. The results show that the fracture mechanism of the specimen under low in-plane and out-of-plane constraints was highly ductile fracture, and the J-R curve was relatively high. The fracture mechanism of the specimen under low in-plane and high out-of-plane constraint was ductile fracture near the initial pre-cracks, and changed to mixed fracture of brittle and ductile fracture away from the initial pre-cracks, and the J-R curve was centered. For the specimen under high in-plane constraint and high or low out-of-plane constraint, the fracture mechanism was mixed fracture but dominated by brittle fracture, and the J-R curve was relatively low.