International environmental issues are complex and depend on joint actions of all states. However, international environmental cooperation lacks a central authority. As a consequence, different attitudes of states lead to many problems in cooperation. Previous studies focus on the interestbased explanation but ignore professional knowledge in environmental cooperation. International environmental issues need to comprehensively analyze the cooperation space with the use of the bifactor of knowledge and interest, and develop a theoretical framework with certainty knowledge and national interest. We apply the framework to analyze the reasons why climate change is difficult to cooperate, and how the two cases of ozone depletion and Mediterranean pollution rely on different cooperation approaches. The comparison of the three cases shows that dynamic evolution of certainty knowledge and national interest jointly affect the space of international environmental cooperation. In the early stage of cooperation, weaker certainty knowledge has no obvious effect, and the cooperation mainly depends on national interest. In the middle and later stages, the increasing certainty knowledge plays a more important role in urging states to fulfil their responsibilities through international initiative pressure and domestic public opinion pressure. The improvement of funding and technology mechanisms and the increase of new technologies and governance benefits make international environmental cooperation easier to achieve.