Abstract: According to the structure characteristics between the partition and positioning plates of the pressure boundary water chamber of nuclear reactor steam generator, the butt welding was conducted on Inconel600 alloy with an asymmetric X-groove design, and the microstructure and hardness of the welded joint were analyzed. The results show that the grain size in the heat affected zone of the welded joint was larger than that in the base metal. The structure of the fusion zone transitted well. The structure at the edge of the weld seam was composed of columnar crystal, growing perpendicularly to the fusion line toward the center of the weld seam, and the structure of the center of the weld seam was composed of columnar crystal and equiaxed crystal. More equiaxed crystals were formed in the remelting zone between the front and back sides of the weld seam. The phase composition of the weld seam consisted of γ phase, γ' interstitial phase and carbides, the same as that of the base metal. The hardness in the weld remelting zone was the highest while that of the heat affected zone was the smallest.