Regarding the forest as a mixture of air and plants, the refractive model is used to obtain the effective dielectric constant of forest, and the correctness is verified by comparing with the experimental result. The parabolic equation model of forest can be improved by introducing the method of effective dielectric constant for forest. Compared with the traditional model, the improved model takes into account the effect of various elements of forest on radio wave propagation and is more suitable for the radio propagation problems with different plant species in different regions. Otherwise the non-uniform mesh technique is introduced and improves the computational efficiency effectively. Finally, in this paper we analyze the effects of various elements of forest, such as the volume content of plant, the gravimetric moisture content, etc. on the radio propagation.