Biomass‐based hydrogel is a promising flame‐retardant material and has a high potential for applications in transportation, aerospace, building and electrical engineering, and electronics. However, rapid vat photopolymerization (VP) 3D printing of biomass‐based hydrogels, especially that of all‐natural ones, is still rare. Herein, a new class of VP 3D‐printed hydrogels with strong covalent networks, fabricating using fully biomass materials and a commercial liquid crystal display (LCD) printer assembled with low‐intensity visible light is presented. Encouragingly, the highly ordered layer‐by‐layer packing structures provided by VP 3D printing technology endow these hydrogels with remarkable flame retardancy, exceptional temperature resistance, advantageous combustion behaviors, and favorable mechanical strength, in particular, giving them a better limit oxygen index (83.5%) than various biomass‐based hydrogels. The proposed approach enables the green design as well as the precise and efficient preparation for flame‐retardant materials, paving the way for the future flame‐retardant materials toward attaining green sustainability.