The Western China Development is confronted with a quite serious and realistic problem on water resources because of the water cycle's district characteristics, the ecosystem's vulnerability and the increasing demand of water resources. Based on the traits and existent problem of water cycle in western China, the key scientific issues of water cycle in western China has been addressed in this paper. Meanwhile, the prospect of water cycle study in the 21st century is also been discussed. The key issues are as follows: In the northwest arid area of China, what are the evolution rules of water resources which are affected by the nature change and human activity? How much eco-demanded water should be distributed on the stand of the national macro stratagem in order to keep the balance of northwest area's ecosystem which is vulnerable in the conflict between nature and human beings? How large is the social economy scale, which can be supported by regional water resources? How to reasonably distribute and manage the water resources to maximum exert its potential capability to boost the sustainable development of this area.