The electricity market environment, the quantified description from power quality synthetic evaluation is a vital basis for Power Quality Evaluation (PQE) and electricity pricing. This paper proposes a method to comprehensively evaluate power quality based on the Maximum Tree algorithm for fuzzy clustering in fuzzy clustering algorithm. First, the power quality evaluation system is formed by selecting single index for power quality evaluation. Then a key set of evaluation index is determined with major factors influencing power quality, which are found by first forming a fuzzy matrix by original data matrix after range transformation, then establishing a fuzzy similar matrix that depicts the relevance of each index, and establishing the Maximum Tree as per max tree construction algorithm. At last, the classification rule can be further acquired from the key set of evaluation index. This rule can sort the power quality of each monitoring point to obtain a reasonable assessment result.