The invention discloses a method for thermally synthesizing a single-crystal bismuth nanowire by solvent in a magnetic field. By utilizing the characteristic of anisotropic growth induced by the magnetic field, the single-crystal bismuth nanowire is produced through a polyhydroxy reduction reaction between sodium bismuthate and glycol. The whole reaction process is performed in the magnetic field, and the Bi nanowire produced by the synthesis has a diameter of 40 nanometers and a length of between 10 and 50 microns. The method is simple, convenient and safe, and has high repeatability, high output, and good product crystallinity; and the bismuth nanowire produced by the synthesis can be widely used in the fields such as physics, chemistry, material, micro-electronics and so on. The method also provides a novel idea and a novel means for materials which are disadvantageous to grow into nanowires.