For the complexity of convective term, the interior heat transfer is important for the engineering testing of heat conduction of fibrous porous materials. In this paper the heat transfer through the body of fibrous porous materials was simulated with finite volume method. By the simulation, it is found that the total heat flux through the body of fibrous porous materials is a linear function of the thermal conductivity when it is measured by the guarded plate, and some constants in the linear function are related with the thickness and permeability coefficient of the sample. The simulated data are employed to fitting the variation curves of the total heat flux with thermal conductivity, thickness and permeability coefficient, respectively. The improved calculating formula of thermal conductivity for fibrous porous materials is established based on the fitting estimation. Through the experimental, it is demonstrated that the improved calculating formula is more accurate than the original one, which is based on the assumptions of single component continuum material and one dimensional heat transfer.