A graft copolymer of betain onto hydroxypropyl guar gum was prepared by using ceric nitrate as initiator. The product structure was characterized though IR. The effects of graft reaction temperature.，graft reaction time, initiator concentration, m（betain）: m(HPG) on thickening properties of product was investigated. It was indicated that monomer transfer rate was 36.5% and the product molecular could be 1.28?06. The product of 0.5% concentration in aqueous solution could be dissolved completely within 20 minutes and viscosity was 75mPas. The solution viscosity kept increasing with the increasing of salt concentration if salinity was less than 10%. No precipitate appeared during the process.