A reasonable power quality evaluation method is not only the foundation to set the price according to power quality, but also the important basis to manage electric energy comprehensively. In order to evaluate the power quality objectively and scientifically, a novel relative similarity degree index is presented to evaluate the grade of power quality and order preference by integrating the grey correlation analysis into the ideal solution (TOPSIS) method. Then it can avoid the case that TOPSIS cannot distinguish samples' disparity due to the equal relative similarity degrees. Furthermore, to achieve a unification of objectivity and subjectivity of weight, a weight factor is determined by combining the G1 with entropy weight method, which is also a foundation for evaluation. A case study demonstrates that the proposed method can elaborate the grade of power quality and make the power quality assessment being more objective, reasonable and comprehensive.