Based on the principles of extenics and its major characteristics of matter-element theory， an analysis is made of the main factors affecting the quality of power supply service. Furthermore， the matter-element analysis model is used to build a quality evaluation index system for power supply service in the following steps: quality level division， construction of matter-element analysis model， calculation of correlation function values， calculation of weights， and determination of membership grade. Empirical analysis show the feasibility of the evaluation method proposed.