The invention relates to a 3D printing method for fiber-grade reactive extrusion. A lactam monomer and/or lactone monomer and an assistant are fused and have a polymerization reaction under the catalyst effect, and target polymer is obtained; the target polymer is subject to fused deposition 3D printing, and then is 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-adaptability 3D printer is adopted, the molding precision is high, the efficiency is high, and the mechanical property of the product is good; a technology for directly making the product from the polymeric monomer is adopted, the production flexibility is improved, 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 condition that due to high polymer degradation, the performance of the product is reduced 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 artware, machines, chemical engineering, instruments, automobiles and the like.