Abstract: There are three questions that should be solved to judge whether an evaluation method is good or not:(2) to set up the principle of the judgment;(2) to build the frame of reference;(3) to design evaluation index system. This paper brings forward a three-layer evaluation principle for the first time: to keep the category, then to keep the rank and at last to keep the distance. The paper also firstly put forward the thought that theories about the mode, the sum of rank numbers, and weighted average should be employed to build the frame of reference to decide the category, the rank and the distance. Grade correlation coefficient and gray coherence should be used as evaluative indexes. Through this set-up method, the evaluated results can be more objective.