A map of a building using through-the-wall radar imaging (TWRI) can be best obtained by detecting and identifying its internal principal scatterers, where estimation of the pose angle of a trihedral formed by the wall-wall-floor structure is important in this application. In this paper, an image-domain based method is proposed to estimate the pose angle of trihedral using a feature called amplitude ratio (AR). The estimated pose angle of a trihedral is determined according to AR. Firstly, the imaging geometry of the radar with a multiple-input multiple-output (MIMO) array and the definition of AR in the echo-domain are described. Secondly, a parametric back-scattering model based on geometrical theory of diffraction (GTD) is applied to analyze AR in the echo-domain when a trihedral is in different pose angles. Thirdly, a virtual aperture imaging model is developed to describe the imaging procedure using MIMO array. Based on the imaging model, the AR of each trihedral can be calculated in the image-domain instead of the echo-domain. Finally, the proposed estimation method is validated by the real data collected in an anechoic chamber.