**3. Methods**

The main methods currently used for eco-efficiency evaluation are the single ratio method, the indicator system method and the model analysis (DEA) method. Although the single ratio method is relatively simple, it cannot distinguish the impact of different environments, give the optimal set of ratios or give decision makers flexibility in their choices. The indicator system method can reflect the level of development and coordination of social, economic and natural subsystems, but it requires artificial weighting, so there is often subjective interference. The use of the model analysis method can better compensate for the above shortcomings, so the data envelopment analysis method is widely used in the empirical study of eco-efficiency.
