High entropy precursors were first synthesized by using a solvothermal method with (Ti, Mo, W, Ta, Nb) transition metal chlorides, ethanol, cetyltrimethylammonium bromide and sodium hydroxide as raw materials, with the effect of NaOH to be studied. Subsequently, (Ti_0.2Mo_0.2W_0.2Ta_0.2Nb_0.2)B₂ powder was derived from the TiMoWTaNb high-entropy precursors and boron carbide by using a solvothermal method combined with a molten salt-assisted boron-carbon thermal reduction process. The effects of reaction temperature, holding time and reactant dosage on phase composition and microstructure of the powder were studied. When the reaction temperature is 1473 K, the holding time is 3 h, the content of NaOH is 64 mmol, n_TiMoWTaNb:n_B4C = 1.0:1.1, and the salt-feed ratio is 5.0:1.0, the highest purity can be achieved. The single-phase (Ti_0.2Mo_0.2W_0.2Ta_0.2Nb_0.2)B₂ powder has a uniform element distribution.