Abstract: Adopting the butt welding method and using NiCrMo-6 electrode as the filler material, the welding experiment of the 9 mm thickness 9Ni steel base metal was carried out. Through optical microscope (OM), scanning electron microscope (SEM) and energy spectrum analysis (EDS), etc. Method Observe and analyze the structure of fusion welded joint. Nanoindentation technology is used to improve the microstructure and mechanical properties of the base metal (BM), coarse-grain heat affected zone (CGHAZ), fusion zone (FB) and weld center area (WM) in the fusion welded joint. The relationship is characterized. Supplemented by the joint in-situ tensile test to determine the weakened area. The results show that: in the columnar structure of the weld, Fe-Cr, Ni-Cr-Fe and other main precipitation phases are dispersed on the austenite matrix, and the heat-affected zone is mainly composed of coarse lath martensite and bainite. The inverted austenite phase between the bars has basically disappeared. The nanoindentation test results show that the welded joint has outstanding resistance to plastic rheological deformation. The coarse-grain heat-affected zone has the highest strength, while the fusion zone has the lowest strength. The tensile fracture observation test further shows that the fusion line area is prone to tearing stress. Fracture is the weak area of the joint.