Abstract: The harmony degree of the ecology-economy-society system was analyzed by using the harmony function and a series of basic theory of evaluating the ecology-economy-society system were established including the efficacy function, harmony function, rank of harmony degree and evaluation index. Four kinds of method were constituted about the harmony function, being the geo-mean method, weighted mean method, variation method and coefficient of variation method, at the same time the rank of harmony degree was analyzed completely. Finally, the harmony theory was validated by a case study of Ziyang District, Yiyang City, Hunan Province. The result is identical with its fact in the way, so the harmony theory is scientific and feasible.