In this paper, αα-acrylic acid was grafted on the surface of polyester fiber by low temperature plasma,and the treated fabrics were dipped in a solution containing ammonium dihydrogen phosphate, then the flame retardance was studied. The optimum processing condition was as follow, the grafting monomer concentration 25%, the grafting reaction time 6 hours, reaction temperature 100 ℃, the concentration of ammonium dihydrogen phosphate solution 25%. The results showed that flame retardance of the modified polyester fabric was obviously improved, and the limiting oxygen index(LOI) reached 29.5.