There have been great changes in the whole electric supply chain due to electric power system reform in China. Power plants are the main blocks of power market'competition. They must reduce production cost and combine internal and external resources to gain continuable competitiveness. Since coal is the most important material for coal-fired power plants and possesses a large portion in total cost, it has been a key strategic issue for power plants to evaluate and select upstream coal suppliers. Data envelopment analysis (DEA) has been widely used in evaluating decision making units' (DMUs') relative efficiency, but it has several limitations as below: being sensitive to errors; excluding qualitative and moderate indices. This paper is based on a 'simulation example to assess the relative efficiency of coal suppliers. In that process, an evaluation index system is constructed, including quantitative and qualitative indices; positive, negative and moderate indices as well. Specifically, fuzzification models are formulated for every kind of indices. With the index fuzzification results, a fuzzy DEA model is constructed, taking qualitative and moderate indices into account, providing a more comprehensive evaluation method than traditional DEA model.