Excellent strength regenerated alginate composite fibers mixed with Poly (ethylene glycol) grafted cellulose nanocrystals (CNC-g-PEG) were prepared through a simple and scalable blending strategy. Firstly, high modulus cellulose nanocrystals (CNCs) were fabricated based on waste cotton fabrics by the chemical treatment. Poly (ethylene glycol) (PEG) with different molecular weight was grafted onto the CNC surface. The composite fibers were prepared from CNC-g-PEG/sodium alginate spinning dope by wet spinning. The CNC-g-PEG/Alginate fibers were characterized using FTIR, XRD, scanning electron microscopy, and mechanical testing. The morphology of the SEM image showed that CNC-g-PEG partially eliminated the defects of the alginate matrix and increased the roughness of the fracture cross-section, constructing material with a denser microstructure. Under the same load, the addition of CNC-g-PEG has better effects on the mechanical properties of alginate fibers than CNCs. The maximum strength and elongation at the break for alginate composite fibers were increased by 56.3% and 81.6% as a result of the CNC-g-PEG reinforcement, respectively. Besides, the salt tolerance and antibacterial activity of the CNC-g-PEG/Alginate composite fibers were improved as the molecular weight of PEG increased. This study provides the possibility for the development of high-performance, green alginate fibers.Graphical abstract