Exploitation and use of transboundary water resources may cause negative effect on the environment of the resources and relevant states. As a section of international law regulating use, protection and management of transboundary water resources, international water law shall ensure that water resources are used equitably and reasonably, significant harm to the resources and relevant states are avoided, and sustainability of water resources is maintained. First well established under domestic law, environmental impact assessment has been incorporated into many sections of international law. As an effective procedural environmental decision-making measure, EIA plays a more and more important role in facilitating implementation of many principles and rules under international water law and in safeguarding sustainability of transboundary water resources. However, EIA obligation and procedure under international water law has not been well established, the implementation of which still relies heavily on domestic law and practice.