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W-3.5Nb alloy was prepared by selective electron beam melting technology (SEBM). The influence of linear energy density on the formation of pore defects was studied. Different types of pore defects were analyzed. The results show that: With the increase of the linear energy density, the defect content decreased, the volume of defects was the lowest (0.01%) at the linear energy density of 1.44 J/mm. When the linear energy density continued to increase, the volume and quantity of defects increased. There were four types of pore defects in SEBM W-3.5Nb alloy: lack of fusion defects, micron dendritic pores, micron spherical pores and nano spherical pores. At the low linear energy density, the large defect (>5000 μm_³) which was the lack of fusion caused by insufficient energy input and insufficient melting depth was dominant; when the energy density increased, the defect size was smaller (< 2000 μm_³), among which the low sphericity defects were the dendrites defect caused by the dynamic flow of molten pool, and the high sphericity defects were micro and nano spherical pores caused by the shrinkage of the melting pool and micro shrinkage between dendrites during solidification.