Abstract: In order to provide reference for engineering application of Nb521 Nb-W alloy large and thin-wall structural components and accumulate basic data of Nb521 Nb-W alloy laser butt weld, laser welding process test of Nb521 Nb-W alloy with different thickness was carried out by presetting assembly misalignment and assembly gap, and effects of assembly condition on formation of weld, microstructure and mechanical properties of laser welded joints were researched. The results showed that Nb521 Nb-W alloy had excellent laser welding processability. Acceptable maximum assembly misalignment and maximum assembly gap of 1.0 mm+1.5 mm Nb521 alloys laser butt weld was both 0.3 mm, which was less than 30% thickness of thinner base metal. Metallographic structure of Nb521 Nb-W alloy laser weld was typical as-cast microstructure, columnar crystals on both sides grew relatively towards weld center through epitaxial solidification, and width of heat affected zone was 0.3～0.4 mm. Assembly misalignment and assembly gap had little effect on microstructure of weld. When assembly misalignment and assembly gap did not exceed the maximum acceptable value, uniform and beautiful formation of weld on both sides could be obtained. Welded joints had good and comprehensive mechanical properties, tensile smples fractured on base metal and showed a typical characteristic of ductile fracture.Highlights: (1) Laser welding process was introduced to achieve butt welding of Nb521 Nb-W alloys with different thickness, and welded joints had good and comprehensive mechanical properties.(2) Effects of assembly misalignment and gap on laser weld of 1.0 mm+1.5 mm Nb521 Nb-W alloys were analyzed, and assembly threshold was achieved.