The invention relates to a ferromagnetic glass fiber, a preparation method, and an application of the same. The glass fiber is composed of the chemical components comprising, by mass, 25-45% of Fe2O3, 20-50% of SiO2, 2-10% of Al2O3, 0-8% of B3O3, 2-15% of CaO, and 1-6% of MgO. The preparation method includes the steps of: preparing a uniformly mixed compound material from pyrite slag, quartz sands, aluminum oxide powder, boric acid, calcium carbonate and dolomite; melting, clarifying, homogenizing and cooling the compound material; and wire-drawing and drying the material. By means of the method, the industrial waste residue, pyrite slag, generated during industrial sulfuric acid production can be completely utilized in one time, so that a problem of waste residue treatment in preparation of sulfuric acid by roasting pyritic in chemical engineering is fundamentally solved, thereby achieving the targets of utilizing waste residues, improving environment and reducing production cost. The method greatly increases additional value of glass fibers and has great economic and social benefits.