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The Nb521 alloy was fabricated by selective electron beam melting to investigate the densification properties. The microstructure, phase, micro-hardness and room temperature tensile properties of the as-build niobium tungsten alloys were tested and analyzed, and the mechanism of selective electron beam melting of Nb521 alloy was also discussed. The results show that the reasonable matching of electron beam melting current and velocity is the basis for obtaining the samples with excellent surface quality and internal metallurgical quality. When the energy input density is 340 J/mm3, the density of the sample can reach 8.78 ~8.79 g/cm3. The microstructure of Nb521 alloy along the deposition direction presents columnar crystal characteristics, X-ray analysis shows a strong preferential growth on the (200) plane. The phase analysis of the formed samples shows that in addition to the Nb matrix phase, there are still a small amount of precipitates calibrated as Nb2C and ZrC. The tensile strength of Nb521 alloy at room temperature is 384 MPa, the yield strength is 307 MPa, and the elongation is 16.5%. The hardness shows no anisotropy with a hardness of 1 500~1 700 MPa.