Target decomposition is a key point for polarimetric Synthetic Aperture Radar(SAR)’s applications，where the four-component decomposition has been successfully applied to the analysis of urban areas and some other complex areas. However，the decomposed volume scattering component is relatively large when the four-component decomposition is applied. It is caused by the volume scattering model which is not good for characterizing totally random scatters. In order to accommodate the targets’ real physical scattering characteristics，a modified four-component decomposition based on a new volume scattering model is proposed in this paper. First，the deorientation process is applied to the coherency matrix. Then，with the new volume scattering model，a modified decomposition algorithm is proposed with a power constraint for avoiding the emergence of negative powers. Finally，the NASA/JPL AIRSAR L-band polarimetric data of San Francisco and the ALOS/PALSAR polarimetric data acquired over Beijing area are used for analysis；the experimental results demonstrate the effectiveness of the proposed algorithm.