Mo alloy was prepared with Al, MoO₃, SiO₂ and B₂O₃ as raw materials by combustion synthesis under high gravity, and the effects of B₂O₃ contents on the phase composition, microstructure and mechanical properties of the prepared Mo alloy were discussed. In a high gravity field with an acceleration of 1000g, the metal melt and ceramic melt obtained by combustion synthesis were rapidly separated. After cooling and solidification, the dense Mo alloy was obtained. The phase composition and microstructure of the Mo alloy were characterized by X-ray diffraction(XRD) and scanning electron microscope(SEM). The XRD results show that with the increasing of the B₂O₃ contents, there was a new phase in the product. And the new phase was composed of Mo₃Si and Mo₅SiB₂. It is found that when the contents of B₂O₃ increased from 0 to 12wt%, the density of the prepared Mo alloy was dropped from 9.76g/cm₃ to 9.35g/cm₃; hardness was increased from 300MPa to 1035MPa and flexural strength was dropped from 711MPa to 460MPa. In a high gravity field, the increasing of the B₂O₃ contents promoted the formation of the new phase which was composed of Mo₃Si and Mo₅SiB₂, and the new phase played an important role in enhancing the Mo alloy.