The acquisition of forest parameters can not only estimate the surface biomass and underlying topography,but also contribute to the study of global carbon cycle and global climate change.Vegetation parameter inversion algorithm with polarimetric interferometric SAR (PolInSAR)is generally based on the two-layer RVoG(random volume over ground)model.However,when the actual vegetation has three-layer structure of canopy,trunk layer and surface layer,the vegetation parameters inversion accuracy will decrease. As the vertical effective wave number difference between the near and far range in the case of airborne SAR system is large,it will bring the system error to the final inversion results.To solve these two problems,this paper proposes an algorithm of three-layer vegetation parameters inversion by fusing the ascending and descending PolInSAR data.The proposed method uses the three-layer RVoG model to correct scattering process of radar echo in vegetation.Then it combines the ascending and descending PolInSAR datasets to weaken the system errors;Finally,we use the non-linear iteration adjustment for tree height inversion.In order to validate the proposed algorithm,two ascending and two descending P-band full polarization SAR data acquired by ESAR airborne platform under the German space agency(DLR)BioSAR 2008campaign are utilized and other three inversion strategies are used for comparison and analysis.The results prove the correctness of the three vegetation model,and the proposed method reduces the system error caused by the vertical effective wave number and improves the precision of tree height inversion.