Abstract: The potentiometric titration and dry-bulk density experiment was done using the sediment of different Ca²⁺ concentration in the Yellow River and the Yangtze River and some conclusions are given as following: ①The surface negative charge of sediment firstly unceasingly reduces and then tends stable with the increasing of the Ca²⁺ concentration;②The influence of the Ca²⁺ concentration on the dry-bulk density can be divided into three phases with the increasing of Ca²⁺ concentration:the first phase is that the dry-bulk density reduces,the second one is that the dry-bulk density increases and gets the maximal value,and the third one is that the dry-bulk density reduces and finally doesn't change;③When the Ca²⁺ concentration is equal,the influence of sediment concentration on the dry-bulk density has different change with the increasing of the Ca²⁺ concentration;④As the Ca²⁺ concentration and the sediment concentration are the same,the dry-bulk density will get bigger when the sediment median size increases,and the sand whose median size is big needs liter Ca²⁺ when its dry-bulk density gets the maximal value;And ⑤ the sediment electrostatic repulsion is different because of the dif erent surface negative charges in the different Ca²⁺ concentration,and the water film force is different,too.Both of two reasons show that the dry-bulk density reflects the above mentioned changes.