Based on the “the theory of entrusted environmental responsibility”, this paper analyses the motivations and reasons for practicing environmental auditing from the perspective of demand and supply. Firstly, this paper uses the scale of the researches on environmental auditing as the sample to explain the demands of environmental auditing. Secondly, this paper uses the announcement numbers of the results of environmental auditing released by CNAO as the sample to explain the supplies of environmental auditing. Thirdly, this paper uses the case of environmental auditing as the sample to explain the interactive relationship between the demands and supplies of environmental auditing. It is concluded that the demands of environmental auditing are strong; the practices of environmental auditing rely much more on the supply capacity of environmental auditing，which is affected by many factors; at present, the key to promote the practice of environmental auditing is to improve the supply capacity of environmental auditing.