A novel method of modifying polyacrylonitrile ( PAN) fibers by grafting of soybean protein ( SP) onto it was studied. The reactant of PAN-g-SP fiber was prepared based on chlorination of the hydrolyzed PAN fiber. The effects of chlorination and grafting conditions on the grafting efficiency were investigated. The grafting efficiency first increases with the increase of the addition of thionyl chloride ( SOCl2), chlorination time and temperature and then levels off. In grafting reaction, grafting efficiency increases at first and then declines significantly with increasing addition of sodium hydroxide (NaOH), grafting temperature and time. The grafted polymer was characterized by FT-IR spectroscopy, X-ray diffraction and SEM images. The results indicated that SP was grafted onto the PAN fiber. PAN-g-SP also exhibits good hygroscopicty and proper mechanical properties.