Cotton fabric was chemically modified by a prepared eco-friendly cationic polymer. The cationic polymer was synthesized by the reaction of dimethylamine and epichlorohydrin using the novel method of three-step polycondensation. And it was characterized by Fourier transform infrared (FTIR) spectroscopy, ¹H nuclear magnetic resonance (¹H NMR) and gel permeation chromatography (GPC). Moreover, the structures of both untreated and treated cotton fabrics were compared and investigated by FTIR, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The dyeing properties of the treated cotton fabrics are discussed. The results indicated that the total dye utilization of treated cotton by salt-free dyeing was much higher than that of untreated cotton by conventional dyeing. And the color fastness properties, the dyeing levelness and the tear strength of treated cotton were all satisfactory and could meet application demands. The environmental hazard caused by the dye wastewater could be decreased greatly in the dyeing process.