Abstract: In order to study the thermal response of aerospace composites in fire, equations of the composite thermal response were established considering the process of thermal decomposition, the explicit finite difference scheme was derived, and the internal transient thermal response and carbonization of glass fiber/phenolic resin composites were studied. The results show that the equations of nonlinear thermal response established and the method adopted can predict the temperature-time progress of glass fiber/phenolic resin composites, and the temperature of the heating surface at 800 s reaches 1048℃ and the back temperature is 226℃, which are in good agreement with the experimental values. With the increment of the depth, it takes longer for the material to reach the thermal pyrolysis temperature, at the same time, the rate of decrement of material density also decreases, and the carbonization process slows down. The residual mass fraction of materials at different depth in the pyrolysis reaction zone is slightly different at the same temperature, i.e. the deeper the position is, the smaller the residual mass fraction, and the higher the degree of carbonization. As time goes by, the proportion of materials participating in the pyrolysis reaction increases, the carbonization range gradually expands, and the thickness range of the pyrolysis layer gradually expands.