The invention relates to a 3D printing method of a fiber-grade flame retardant product based on reactive extrusion. According to the 3D printing method, polymerization reaction is performed on a lactam monomer and / or a lactone monomer and an assistant and a plasticizer after fusing under the action of a catalyst, and a flame retardant is added before polymerization or during polymerization to obtain a target polymer; then the target polymer is subjected to fused deposition 3D printing, and then is subjected to thermal treatment, thus obtaining a final product. According to the invention, a high-precision high-adaptation 3D printer is adopted, the forming precision is high, the efficiency is high, the mechanical property of a product is good, and the dispersibility of a functional component is good; and a process of preparing a product directly from a polymeric monomer is adopted, and therefore, the production flexibility is improved, and the conditions of edge warping of nylon, polyester and other semi-crystalline polymers caused by crystallization and large temperature difference, and even failure in printing can be avoided. The method can widen the species of fused deposition 3D printing materials, can improve the quality of products, and is wide in application in the fields of automotive materials, engineering plastics, building materials and the like.