The reasonable and comprehensive evaluation of power quality of power system is an important basis for scientific detection of power quality and effective management and control. Therefore, this paper proposes a power quality comprehensive evaluation method based on cooperative game theory and improved TOPSIS. Firstly, an index system for quantitative evaluation of power quality is established, which avoids the shortcoming of single weighting method. Then, by using the cooperative game theory, the optimized weights of various evaluation indexes are obtained. Finally, the improved TOPSIS method is adopted to evaluate the power quality, and the magnitude of the relative proximity is used for sorting and comparing. After the algorithm is programmed by C++, the power quality of a power supply area is evaluated comprehensively. The results of the example show that the optimal weights determined by the cooperation game theory take into account both subjective and objective effects, the evaluation method based on the improved TOPSIS can effectively evaluate the power quality comprehensively. This work is supported by Science and Technology Project of State Grid Corporation of China (No. 2016KJ0325).