The epoxy resin (F51) was modified by acrylonitrile butadiene rubber (CTBN) to improve its toughness. The structure and properties of the modified resin were characterized by SEM, FTIR, TG and mechanical facilities. The modified mechanism was discussed. The ablative properties of the low density material which made by the modified epoxy resin was checked by gas jet and wind tunnel experiments. The results show that after the modification of CTBN, the molecular chains have a certain degree of cross-linking. The tensile and bending properties of resin matrix decreased, but the toughness was enhanced. Thermal stability is greatly increased. At 800℃, the rate of the residual weight of the toughening F51 increased to 54% from 23%. The shear resistance and erosion resistance of low-density ablative materials have been enhanced, and the stability of the carbon layer has been improved, and the ablative properties have been improved.