The invention relates to a fiber-scale reactive extrusion 3D printing method including the steps of: a) fusing a lactam monomer and/or a lactone monomer, and an additive and an ionic liquid, and performing a polymerization reaction under the effect of a catalyst to prepare a target polymer; and b) performing fused deposition 3D printing to the target polymer and performing thermal treatment to obtain a finish product, wherein the thermal treatment is carried out at 100-180 DEG C for 3-60 min. In the invention, a high-precision high-adaptability 3D printer is employed, so that the method has high shaping precision and efficiency and the finish product has good mechanical performance. A process, in which the polymerization monomers are directly processed to obtain the finish product, is employed, so that the method is improved in production flexibility and also avoids the problems of edge warping and even printing failure since semi-crystallized polymers, such as nylon, polyester and the like, are crystallized and are large in temperature difference, thereby avoiding reduction of performance of the finish product due to degradation of the polymers. The method increases the types of fused deposition 3D printing materials, improves quality of the finish product, and has wide applications in the fields of crafts, machines, chemical engineering, instruments, automobiles and the like.