Abstract: In view of the coarseness of grains in microstructure of pure tungsten after selective electron beam melting (SEBM), B was selected as the grain refinement element. Besides, the influence of forming process parameters on the densification, microstructure and mechanical properties of W-B alloy was explored. It is found that when the forming process parameters are I=15 mA, v=300 mm·s-1 and E=1200 J·mm-3, the density of W-B alloy samples is the highest, and the pores and unfused powders are the least, the microcracks were basically suppressed. The constitutional undercooling caused by the introduction of element B effectively refined the grain size of W formed by SEBM, and the microstructure changed from coarse columnar crystal to fine columnar dendrites with an average size of about 8.26 μm. The highest hardness of W-B alloy is 672 HV, and the highest ultimate compressive strength is 1598 MPa. The fracture mechanism is mainly cleavage fracture.