Abstract: In the ultra-low altitude countermeasure scenario, the multipath effect seriously affects the detection accuracy of the ultra-low altitude radar. The Brewster Angle characteristic can be used to reduce the multipath effect effectively. The actual ground environment is complex and changeable, in which the multiple scattering effect of vegetation and the coupling effect between vegetation and ground make the scattering characteristics of vegetated ground different from that of random rough ground. The study of electromagnetic scattering characteristics of vegetated ground still faces many challenges. Aiming at the multipath effect of ultra-low altitude detection under vegetated ground background, this paper starts from the perspective of electromagnetic scattering modeling. Using a combination of Kirchhoff approximate (KA) and integral equation model(IEM), a facet-based modified two-scale model (FMTSM) with IEM advantages is used to model rough ground electromagnetic scattering. Compared with the traditional two-scale model (TSM), the accuracy is higher and the application range is wider. Using FMTSM method, the Brewster Angle characteristics of rough ground are analyzed. The effects of incident frequency, correlation length and root-mean-square height on Brewster Angle characteristics of rough ground are discussed. On this basis, combining FMTSM with vector radiative transfer theory(VRT), FMTSM-VRT is used to calculate the electromagnetic scattering characteristics of double-layer vegetation under VV polarization, and its Brewster Angle characteristics are analyzed. The variation of Brewster Angle under different incidence frequency, vegetation height and leaf density is investigated. The Brewster Angle of vegetated ground decreases first and then increases with the increase of incidence frequency. With the increase of vegetation height and leaf density, it shows a decreasing trend. The research results of this paper will provide a theoretical basis for effectively weakening the multipath effect in ultra-low altitude early warning system.