Biomass-based supramolecular hydrogels are widely used in the biomedical field due to their favorable biocompatibility and outstanding mechanical properties. However, the preparation of injectable polysaccharide-based hydrogels has proven to be a significant challenge. Here we have employed a simple poor-solvent strategy to prepare alginate supramolecular hydrogels via a hierarchical self-assembly process, including micellization, micelles alignment to form nanofilament, and nanofibrils formation. The alginate supramolecular fibrillar hydrogels exhibit excellent mechanical properties and shear recoverability, meeting the requirements of injectable hydrogels. Furthermore, the presence of alginate and its fibrillar structures imparts superb hemostasis properties and the inherent biocompatibility to these hydrogels. Therefore, this simple and intriguing approach has the potential to develop polysaccharide-based hydrogels for hemostasis in wound within the biomedical fields.