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Carboxymethyl chitosan (CMCS) has good biocompatibility, biodegradability and water solubility. This work investigated a nanocellulose fibrils reinforced CMCS composite hydrogel with giant network structure and quick gelation formability. The microfibrillated cellulose (MFC) with diameter in nano scale and length in sub-micron scale was obtained through high-pressure homogenization. The hydroxyl groups on the MFC nanofibrils surface were partly oxidized into aldehyde groups. The dialdehyde MFC (DAMFC), remaining the fibril morphology and fine three-dimensional network structure, was mixed with CMCS. Due to the Schiff-base reaction between the aldehyde groups on DAMFC surface and amino groups on CMCS chains, a giant network was formed in DAMFC/CMCS. As a result, CMCS composite hydrogel could be formed in short time, although pure CMCS could not form hydrogel. In the composite, DAMFC acted as a reinforcing agent as well as a macroscopical crosslinking agent. The structure and properties of the composite hydrogels were investigated. The compression strength and the work of fracture of the hydrogel was increased dramatically as the DAMFC content increasing from 5 to 15 wt%, an increase of 330% and 338%, respectively. SEM images of the hydrogel showed a fine three-dimensional porous structure, which may provide a promising platform for tissue engineering scaffold. Hydrogels composed of natural macromolecules, having good biocompatibility, are widely used in tissue engineering. This work evaluated a dialdehyde microfibrillated cellulose (DAMFC) reinforced carboxymethyl chitosan (CMCS) composite hydrogel. Pure CMCS molecular chains entangled with each other, but cannot form hydrogel. In DAMFC/CMCS system, a giant network can be rapidly formed due to the Schiff base reaction between the aldehyde groups on DAMFC nanofibrils surface and amino groups on CMCS molecular chains. DAMFC/CMCS hydrogel presents porous structure and drastically improved mechanical strength. Unlabelled Image [ABSTRACT FROM AUTHOR]