The invention discloses a digital method for rapidly evaluating the loss severity of alfalfa diseases and insects. According to the technical scheme of the method, firstly, the focal regions and the non-focal regions of alfalfa leaves are analyzed and then the common disease degree and the RGB color component law thereof are generalized. After that, a color decision-making tree component system for the focal regions and the non-focal regions of alfalfa leaves is established accordingly. The visual processing flow of digital pictures is integrated as one, so that the method is suitable for non-computer professionals to use. At the same time, the functions of batch processing, data exporting and the like are realized, so that the method can be used for processing volume data. Based on the geometric projection imaging rule, the inscribed circle calculation manner is adopted, so that the geometric deformation avoided. The calculation accuracy is improved. Moreover, binary pictures for the healthy regions and the non-healthy regions of alfalfa leaves are constructed, and then the pictures are displayed and compared. In this way, the method can be applied to alfalfa leaves of smaller than 0.5 m in both length and width, with the diameters of the focal regions thereof to be larger than 1mm, and the area statistical precision of the focal regions thereof to be larger than 95%.