Abstract: An intumescent flame retardant 3, 9-dichloro-2, 4, 8, 10-tetraoxa-3, 9-diphosphaspiro-[5.5] undecane-3, 9-dioxide (SPDPC)-hexamethylendiamine copolymer (PSPHD) molecules containing active end-group were grafted onto the sepiolite (SEP) surface via condensation between acryl chloride of PSPHD and hydroxyl groups existed on sepiolite surface successfully. The modified sepiolite were characterized by SEM, TEM, XPS, FTIR, XRD, TGA. SEP/LDPE and PSPHD-SEP/LDPE composites were prepared by melt blending. SEM and TEM photographs illustrate that the surface morphology of sepiolite changes rapidly. The results of XPS, FTIR and XRD show a grafting modification effect on the structure of SEP, but no destruction. The results of TGA and DTG confirm that there really exists a grafting reaction between PSPHD and SEP, the thermal degradation process of SEP change through graftion. The limiting oxygen index (LOI) test results indicate that the flame retardancy of LDPE can be improved with the addition of PSPHD-SEP or SEP, the LOI value of PSPHD-SEP /LDPE composite reaches 23 with 5% mass fraction of PSPHD-SEP.