The numerical reconstruction scheme of equivalent electromagnetic parameters for composite material is proposed based on the finite difference time domain（FDTD） method. To study the relationship between equivalent electromagnetic parameters and the bulk density of mixed medium, the reflection coefficient and transmission coefficient were firstly calculated by the FDTD method, and then the equivalent electromagnetic parameters were reconstructed by genetic algorithm and propagation matrix method under different blending ratios. When the blending ratio of composite material is low or high,the equivalent electromagnetic parameters calculated by the equivalent constitutive parameter formulas and numerical reconstruction scheme agree well with each other. The results are somewhat different only for the middle blending ratio. The results show the feasibility of the proposed numerical scheme for the reconstruction of equivalent electromagnetic parameters of composite material.