The diluent NaCl was added into the W-B₂O₃-Mg system to prepare submicro tungsten boride powders via salt-assisted combustion synthesis. The products were analyzed by XRD, SEM and EDS, and the effect of NaCl content on phase composition, morphology and average particle size of the products were studied. Results show that the combustion synthesis products before leaching mainly consisted of W₂B₅ and MgO, and the morphology of particles was similar to ellipsoid when without adding NaCl. After adding NaCl, combustion synthesis products before leaching mainly consisted of W₂B₅, MgO, WB and B, and the morphology of particles were irregular and particles were slightly reunion together. The products after leaching were composed of small size tungsten boride particles, the average grain size decreased with the increasing of NaCl contents, when the content of NaCl k=20, the smallest particle size was 0.85μm. The purity of products after leaching was higher, the content of W and B were up to more than 98%. The biggest content walue of W and B was 98.51% when the content of adding NaCl was 5%.