The compact polarimetric (CP) synthetic aperture radar (SAR) can alleviate the drawbacks of the fully polarimetric (FP) SAR and provide more target information as compared to the conventional single polarimetric and dual polarimetric mode. However, the present decomposition methods for CP SAR have the problem of overestimation of volume scattering (OVS) among urban areas, especially the obliquely oriented buildings (OOBs). The OVS can cause considerable overestimation of volume scattering which leads to the misinterpretation of the scattering mechanism. In this paper, a new descriptor of OOBs for the CP SAR is introduced to reduce the volume scattering in the urban areas. And a new model-based decomposition method, which is applicable for both the π/4 mode and the CTLR mode, is proposed based on the descriptor. The Radarsat-2 FP SAR data is used to simulate the CP SAR data to validate our algorithm. The proposed method improves the double-bounce scattering component in different types of urban areas by about 10%.