Abstract: Tree gum is mainly composed of natural heteropolysaccharide glycoside. And many researches were developed to made oilfield chemistry by it. Compared with bean gum of fracturing thickening agent, tree gum has better performance of fermentation resistance, so it is quite suitable to apply in the circulative drilling fluid. Based on home-made tree gum, a modification sample of PG tree gum was made through graft method. Analyzed its effect on viscosity and filtration property to drilling fluid at different reaction conditions, such as reaction temperature, time and mass ratio, optimized reaction conditions by orthogonal tests and error analysis. Finally, some performances of PG tree gum and its derivative PG-2 gum in drilling fluids were tested in laboratory, su ch as viscosity enhancing, filtration reducing, lubricating mudcake and inhibiting hydration and swelling of clay.