The invention relates to a functional fiber-grade 3D printing method based on reactive extrusion. A lactam monomer and/or lactone monomer, an assistant and a plasticizer are fused and have a polymerization reaction under the catalyst effect, a functional agent is added before polymerization or in the polymerization process, and target polymer is obtained; the target polymer is subject to fused deposition 3D printing, and then subject to heat treatment, and a final product is obtained; and according to heat treatment, heat preservation is carried out for 3 min to 60 min at the temperature ranging from 100 DEG C to 180 DEG C. A high-precision high-adapting 3D printer is adopted, the molding precision is high, the efficiency is high, the mechanical property of the product is good, and functional components are good in dispersibility; a technology for directly making the product from the polymeric monomer is adopted, the production flexibility is improved, and meanwhile, the condition that due to crystallization and the large temperature difference of semi-crystalline polymer of nylon, polyester and the like, edge warping is caused, and even printing cannot be carried out is avoided; and the kind of the fused deposition 3D printing material is widened, the quality of the product is improved, and the method has the wide application to the fields of automobile materials, engineering materials, structural parts and the like.