The invention discloses a metal fiber brush and a preparation method thereof. The brush comprises conductive metal fibers and filling metal fibers. The length of the filling metal fibers is shorter than the length of the conductive metal fibers, one end of each filling metal fiber is flush with one end of each conductive metal fiber, the conductive metal fibers and the filling metal fibers are uniformly arranged along a length direction, and after compacting molding, the filling metal fibers and the conductive metal fibers are integrally wrapped by a metal wrapping sleeve. The preparation method comprises: winding the conductive metal fibers and the filling metal fibers together on a wire reel to form a metal fiber ring; and cutting the metal fiber ring, after the compacting molding, immersing one end of the metal fiber ring in an acid solution or aqueous alkali, and dissolving the filling metal fibers to obtain the metal fiber brush. The metal fiber brush is compact and reasonable in structure, high in fiber bonding strength, simple in preparation process, large in current carrying capability, small in contact resistance, long in service life, fast in sliding speed, small in heat productivity and small in debris amount, is suitable for industrial production, can replace a conventional carbon brush or a conventional metal fiber brush, and can satisfy the application demand for a large-power high-precision conductive slip ring.