The processes of water resources exploitation and utilization can be divided into three stages by water resources transformation, and the history, present situation and future trend of water resources development in piedmont areas around high mountains of arid northwestern China. The three stages are: the stage of surface water development (the first stage), the stage of comprehensive development of surface and ground water (the second stage) and the stage of economical development of water rare (the third stage). The three stages link each other and show the law and processes of water resources exploitation and utilization associated with social and technological progess. The economical water policy should run through the three stages. On this point, however, the third stage differs from the others, particularly, refering to irrigated agriculture. The third stage has more progressive significance because it breaks the traditional ideas on water resources development. According to our investigation and calculation, under present conditions of water resources development, the net used water is about 160 × 108 m3, accounting for 18% of the total water resources of northwestern China. The water resources have not ben fully develop. If the first stage is finished, the exploitable water can be increased by 91%. After the second stage, furthermore, it can be increased by 216%.