An polyacrylonitrile fiber is one that contains at least 85% by mass of acrylonitrile comonomer in the polymer chain. The nitrile group (-C≡N) in polyacrylonitrile fiber is converted to carboxyl group(–COOH) after hydrolysis in NaOH aqueous solution.This research explores the hydrolysis of copolyacrylonitrile fiber in sodium hydroxide solution and characterization by chrome yellow method. The optimum hydrolytic technology is sodium hydroxide mass fraction is 12%, the time of hydrolysis is 10min, the temperature of hydrolysis is 80°C. In addition, the surface morphology of the polyacrylonitrile fiber and the modified polyacrylonitrile fiber were observed by SEM. The strength of the hydrolysed fiber is decline and the moisture regain is increased compared with the polyacrylonitrile fiber.