The invention discloses a modified polyurethane color fixing agent of basic ion, which is prepared by PEG600, PPG2000, TDI, DMPA, methyl ethyl ketone, calcium trisodium diethylenetriamine pentoacetate, epichlorohydrin, acetone, isopropanol, sodium bisulfite, water and glacial acetic acid. The process for preparation comprises polyurethane prepolymerization reaction, modified chain extending reaction, blocking reaction, emulsion reaction, removing solvent and the like. The modified polyurethane color fixing agent of ion of the invention can form net-shaped cross bonding with woven fabric, and the invention adopts sodium acid sulfite to be blocking agent, which guarantees that when in application, enough isocyanate bonds are supplied to form chemical crosslinking with hydroxyl group and carboxy group with cloth surface, the invention improves various color stabilities of dyed fabric, meanwhile, the invention keeps soft hand feeling of fabric.