This study focuses on the preparation of functional cotton with high and durable antibacterial activity by in situ formation of Ag nanoparticles (NPs) onto cotton fabric derived from phytic acid-Ag complex. The route can be divided into two simple steps, adsorption of silver ions onto cellulose matrix with phytic acid as a capture agent and subsequent reduction of Ag+ to Ag NPs by sodium borohydride. The successful deposition of Ag NPs on cotton fabric was verified by SEM, EDS and XPS. The bacterial reduction rate against E. coli and S. aureus for the as-treated fabrics was above 99%, even after 10 laundry cycles. The phytic acid was found of benefit to distribution and bonding of silver on the cotton fabric, which might lead to the enhancement of antibacterial property and durability against wash. This study may provide a green, novel and simple strategy to manufacture Ag-based antibacterial cotton for potential applications in textile industry.