In order to realize the quantitative guidance for the focusing of dish concentrator mirror unit, it is essential to establish the unique quantitatively corresponding relationship between the mirror unit posture and the focal spot feature. We have presented the equivalent posture error by combining rotation and translation movement of the mirror unit, and the focal spot flux density distribution model under the posture error condition is established based on the ray tracing method. The concentration ratio threshold is applied to extracting the boundary characteristics of the focal spot, and the least-squares method is used for elliptical fitting of the boundary characteristics which represents the distribution characteristics of the focal spot. The factors influencing the focal spot characteristics are analyzed, including the direct normal solar irradiation, concentrator ratio threshold, and mirror unit posture error. The results show that the ellipse geometry of the focal spot under different mirror unit posture error conditions can be obviously distinguished, and it is feasible for the mirror unit posture inversion, which provides reference for the construction of the subsequent mirror unit focusing system.