LaB6-C/C preform containing 9.73% wt.% LaB6 was prepared by slurry impregnation combined with resin infiltration pyrolysis, and then LaB6 modified C/C-ZrC-SiC composites were obtained by reactive melt impregnation (RMI). The microstructure and ablative behavior of the composites were studied, and the mechanism of LaB6 on the ablation properties of the composites were investigated. The results show that, after oxyacetylene ablation at the heat flux of 2380 kw/m2 for 120 s, the mass ablation rate and linear ablation rate of LaB6 modified C/C-ZrC-SiC composites are 1.05×10-3 g/s and 2.17×10-3 mm/s, which are 74.8 % and 61.9 % lower than those of unmodified C/C-ZrC-SiC composites, respectively. During ablation, LaB6 is oxidized to La2O3 and B2O3. The solid solution and chemical reaction between La2O3 and ZrO2, coupled with the effect of liquid B2O3 to promote mass transfer in solid phase reaction, make the surface of the material form a large area of continuous and stable ZrO2-La2Zr2O7-La0.1Zr0.9O1.95 molten protective layer, which is the main reason for the excellent ablation performance of the material.