Abstract: By adding high molecular weight flocculants intohigh turbid concentration suspensions with Yellow-River sediments,and by using settling technique and image analysis technology.The paper discusses bridging flocculation mechanism and development trends of aggregate sizes under different concentration of high molecular weight polymer.Meanwhile,the effects of various concentrations on fractal structure of the sediments aggregates are analyzed,quantitatively,by using the parameter "fractal dimension" characterizing fractal properties.Conclusions are drawn as follows: ①Bridging flocculation mechanism is dominant and charge neutralization mechanism is secondary for flocculation functions of cationic polymers to sediments particles;②Sizes distributions of bridging flocculated silts aggregates present generalized normal distribution under different concentrations of the polymer.There are three regions for sizes distributions of silts aggregates at different flocculation stages;③There is an optimum value about the concentration of a high molecular weight polymer.Under the proper concentration,flocculation efficiencies are ideal: the structures of bridging flocculated aggregates are compact,their settling velocities are great,the residual turbidity of the supernatant is low under the quiescent settling experiment and the fractal dimension value is high.