Abstract: The water renewal is the basic aspect of water cycle and water balance.On the condition that the water inflow velocity is not equal to the water outflow velocity,a revised exchange period model for water bodies and its numerical algorithm are set up.A case study of the three reaches of the upper reach of the Yellow River is made,including Lanzhou-Xiaheyan, Xiaheyan-Shizuishan and Shizuishan-Toudaoguai,to demonstrate the water exchange period in each reach in the natural flow situation and the reservoir regulation in the past years during the flood season and non-flood season.The results are as follows: (1) In the natural flow condition,the exchange period of three reaches during the non-flood season in the past years is longer than that in the flood season in the corresponding time.But it is not same in the condition of reservoir regulation.(2) The water exchange period of each reach in the condition of the reservoir regulation is longer than that in the natural flow condition in either the flood season or the non-flood season; it means the reservoir regulation prolongs the water exchange period in the lower reaches,slows down its cycle velocity of water quantity,and restricts the water renewal quantity of the correspond reaches.