Abstract: Water quality has much influence on the dry sperific weight of sediment in natural river, the primary influence is from the metal cations in water. This paper studies the influence of Na+,Ca2+,and Al3+, which are familiar in natural river, on the dry specific weight of sediment by the experimentation of sediment settling in water. The results of the experimentation show some conclusions as follows. ① The influence mechanism of the metal cations on the dry specific weight of sediment is similar, but the influence of high valence ions is bigger. ② The influence of the concentration of  cations on the dry specific weight of sediment is biggest, and the change of dry specific weight may be divided into three phases with the concentration of cations increase: first phase is that the dry specific weight is reduced; the second phase, the dry specific weight is increased and reached the maximal value; the third phase, the dry specific weight is reduced once again and, finally, is remained constantly. ③ when the concentration of cations is equal, the influence of sediment concentration on dry specific weight  has different changes with the concentration of cations increase. ④ As the concentration of Ca2+ and the sediment concentration are the same, the bigger the median size of sand, the heavier its dry specific weight, and the number of cations of larger median size, which is needed to reach the maximal value of dry specific weight, is less than that ofsmaller median size.