Abstract: Nickel-based welding material 52M is widely used in important parts of nuclear power plants because of its excellent resistance to intergranular corrosion and corrosion resistance. Welding consumables 52M was used to connect the dissimilar metal low-alloy steel SA508Gr.3Cl.2 and Inconel690 alloy by shielded metal arc welding. Combined with OM and SEM, the microstructure near the fusion boundary, composition distribution and precipitates were analyzed to investigate their effects on the hardness and crack formation of the deposited metal. The results show that I type boundary, II type boundary, semi-island structure and island structure are formed near the fusion boundary of the buffer layer. The eutectic reaction of S, Nb and Mo elements in local grain boundaries causes the liquefaction, which results in the formation of a few of solidification cracks in the deposited metal. Due to the solid solution strengthening effect of Nb, Mo, Cr and other elements, the microhardness of the deposited metal is higher than that of the base material at both sides.