Water security issue is not only an international hot topic,but also big challenges and problem of China on guaranteeing Chinese water security and realizing its sustainable development. This paper gives a definition of water security and addresses the important issue on new advantages of water security under a changing environment on both China and international,which includes water security assessment and new progresses of water security issue related to global change and its impact. It is emphasized on basic studies of hydrology and water resources related to climate change impact and water security issue,also interdisciplinary approach between natural sciences and social science,and water and the future of humanity. Some of suggestions and countermeasures are given as adaptive water management to changing environment.