The invention relates to ultrahigh-temperature high-entropy boride ceramic powder and a preparation method thereof, and belongs to the technical field of ceramic materials. The ceramic powder disclosed by the invention is of a single-phase hexagonal crystal structure; the chemical formula is abbreviated as TMExB2, TME is at least four of Ha, Zr, Nb, Ti, Ta, Mo, W and Cr, and the product of the number of elements in TME times x is 1; the ceramic powder can keep a stable single-phase structure mainly by regulating and controlling the components of the ceramic powder, the problems of complex interfaces and stress caused by multiple components are solved, and the ceramic powder has excellent mechanical properties. Furthermore, the high-entropy boride ceramic powder is synthesized by an induction plasma spheroidizing sintering process for the first time; the method is simple in process and short in period, the high-entropy boride ceramic powder with certain particle size distribution, compact surface, high degree of sphericity, good fluidity, no element segregation phenomenon and good chemical structure stability can be obtained by regulating and controlling process parameters, and themethod has a good application prospect.