The invention discloses a boron-containing silicon carbide fiber and a preparation method thereof. The preparation method of the boron-containing silicon carbide fiber comprises the following steps: in a closed reaction container, carrying out synthesis reaction on polysilacarbosilane and a boron-containing monomer at high temperature and under high pressure to generate a boron-containing polycarbosilane crude material, wherein the polysilacarbosilane is a low-molecular product of polydimethylsiloxane subjected to high-temperature cracking, is in a liquid state at room temperature, and has a molecular weight less than 1000 g/mol; dissolving and filtering the boron-containing polycarbosilane crude material to obtain a spinning-grade boron-containing polycarbosilane precursor; and carrying out melt spinning, non-melting, high-temperature sintering and sintering treatment on the spinning-grade boron-containing polycarbosilane precursor to obtain the boron-containing silicon carbide fiber.Compared with the prior art, the preparation method provided by the invention is simple in process and easy to operate, and the synthesized boron-containing polycarbosilane precursor has the advantages of good spinnability, convenience in adjustment of boron content and the like.