High-purity Ti0.2Zr0.8B2 solid-solution nanopowders were successfully synthesized via a molten salt assisted borothermal reduction at 1323-1373K using ZrO2, TiO2 and amorphous B as starting materials. The Ti0.2Zr0.8B2 solid-solution nanopowders synthesized at 1323K show the largest specific surface area of 12.24m(2)/g and the lowest equivalent average particle size of 86nm. Meanwhile, they exhibit the high compositional uniformity and the good single-crystal hexagonal structure. This study provides a new method to synthesize the high-purity solid-solution nanopowders of the transition-metal borides.