Flax fiber (FF) was grafted chemically with maleic anhydride (MA) that gave MA-FF, then treated with sodium carbonate solution that gave the flame retardant modified FF with Na+, expressed as Na2CO3-MA-FF. The appearance of carbonyl peak in FTIR proves that MA was grafted to FF. The grafting degree (Gd) of MA on FF was determined by acid-base titration. The flame retardant effect of Na+ on FF was evaluated by LOI and vertical burning test. The FF crystallinity was analyzed by XRD. The results show that when MA and FF react for 2.4h in DMF with the mass ratio of 48:60 under 105 ℃, the Gd can reach 71~75%. The flame retardancy of FF increases with the increase of Na+ content. The LOI value of 20-Na2CO3-MA-FF（MA-FF was treated with 20 g/L Na2CO3 aqueous） was increased from 20% to 28.9%, the afterflame time decreased from 41 s to 0 s, damaged length decreased from 30 cm to 8.3 cm. After MA grafting, the FF crystallinity increased from 66.7% to 75.0%, the crystallinity of 20-Na2CO3-MA-FF was slightly reduced to 71.8%, but the 2θ angles have no change, are all still 14.8 ° , 16.7 ° and 22.8 °.