The invention relates to a rare earth coupling agent, a preparation method and the application of the rare earth coupling agent in treatment of glass fiber reinforced polyformaldehyde composite material; the rare earth coupling agent comprises the following raw materials in parts by weight: 100 parts of rare earth oxide, 100 parts of inorganic acid solution and 150 parts of polyvinyl alcohol water solution, wherein the rare earth oxide is one or two of lanthanum oxide and cerium oxide. For the application of the rare earth coupling agent in the treatment of the glass fiber reinforced polyformaldehyde composite material, a composite material can be prepared by steps of melting glass fiber treated with the rare earth coupling agent and polyformaldehyde (POM) resin, mixing with antioxidant, formaldehyde absorbent and formic acid absorbent and finally pelleting; compared with a POM matrix, the composite material has the advantages that the strength is improved by 2-3 times, the bending strength is improved by 2-3 times, the heat distortion temperature is increased by 60-65 DEG C, and the coefficient of thermal expansion is reduced by 60-80%.