According to the experimental phenomenon that carbon layer thickness is greater than the insulator matrix loss thickness, a bulk ablation model considering insulator expansion phenomenon with double regions is established. It's believed that expansion is produced during pyrolysis process and the formation rate of carbon layer is 2 times faster than the lapse rate of matrix. At each time step, the interface position of carbon layer and the matrix is determined according to the relationship between local temperature and insulator pyrolysis temperature, and the method of remeshing and data exchange coupling calculation are developed at the same time. The ablation rate and carbon layer porous structure obtained by the double regions bulk ablation model agree well with the experimental results, which proves that the model can basically describe the ablation details of EPDM(ethylene-propylene-diene monomer) insulator.