According to the aerodynamic-thermochemical ablation model, the ablation and thermal protection mechanism of functionally gradient materials are investigated, which are derived by coupling the ablative calculation with heat conduction calculation. Then the ablative ratio formula of C/C/A12O3, which is a type of novel materials, is deduced. At last, the computed results are compared with the test results, according which the effect of the compositions gradiently distributing upon the thermal field under ablative boundary is analyzed.