So far there is still no mature theory for international cooperation in science, and no special researches conducted on international cooperation theory in environmental science yet. This study argues that International cooperation on environmental sciences should be viewed as the allocation of global resources. Environmental Sciences, driven by both the intrinsic scientific factors and the extrinsic social factors, promote the globally scientific and social resources to benefit the development of environmental science itself. At the same time the allocation of scientific resources should follow ‘The Principle of Selection of Best Elements’, and the allocation of social resources should follow ‘The Principle of Least Effort’. Both two kinds of factors and two kinds of principles together constitute the mechanism for international cooperation in environmental science.