Mapping, monitoring and parameter inversion in the mountain areas covered with dense vegetation as well as cloud and mist is always a complicated subject. Synthetic aperture radar (SAR) is an effective sensor of great potential to solve this problem. In this paper, a high resolution P-band airborne SAR system is developed, which is the first quantitative airborne P-band SAR system for business operation in China. The inherent signal ambiguity phenomenon for wide beam radar system is discussed, and the solution is suggested and validated with this radar system. Then a flight campaign is launched with this P-band SAR system in the Southwest China. The defocusing introduced by the dramatic terrain undulations is analysed detailedly. By co-registering the external digital elevation model (DEM)to the SAR image and motion compensation, the refocused quality of the polarimetric SAR images satisfies the user's requirement. After polarimetric and radiometric calibration, the polarized composite and geocoded image products are obtained for further studies. With this P-band airborne SAR system, the repeat-pass interferometric data are expected to be collected in the future for the vegetated mountainous area, and it will play more important role in the fields of topographic mapping and deformation monitoring.