Abstract: Taking the ENi Cr Mo-6 Ni based alloy electrode as the filler material, the butt welding test of 9%Ni steel base metal was carried out. Nanoindentation technology was used to study the relationship between the microstructure and mechanical properties of the base material, coarse grain heat-affected zone, fusion zone and welding center area of the welded joint. The weak area of the joint was determined through the tensile test. The results show that the joint has good plastic toughness. The coarse grain heat-affected zone has the highest strength, the chromium-rich precipitation phases Fe-Cr and Ni-Cr-Fe are dispersed on the austenite matrix, and the inverted austenite between the laths basically disappears. Combining the analysis of structure and mechanical properties, it is found that the cracks start in the fusion zone.