Abstract: The cations in natural river would influence the velocity of sediments especially the fine sediment.The main reason of settling velocity affected by cations is the flocculation of fine grain sediments.On the basis of the experimentation of sedi ments settling in water with differentions concentration,the relationship between settling velocity and the positiveions concen tration is given.The positiveions include Na⁺,Ca²⁺,Mg²⁺ and Al³⁺ etc.The way used in the experimentation is called the pipette way.The results of the experimentation show that the press can be divided into two phases: ①The settling velocity will increase when the concentration of positively chargedion increase; ②If the concentration of positively chargedions are excess of some critical,the settling velocity would not be change,or changed a little as the concentration increase.When different positively charged ions have the same concentration,the ion with higher valency has more influence on sediments settlement Analyzing the characteristic of hydrochemistry in accordance with the data from the Yangtze River and the Yellow River,we can know that it is necessary to consider the effect on sediment settling velocity by the concentration of cat ions on condition of the ion concentration in the nature river.