Abstract: On the basis of a study of water-sediment movement in the Lingdingyang sea area,back silting characteristics of the trial dredged-trough of the immersed tube tunnel of Hong Kong-Zhuhai-Macao Bridge(HZMB) are analyzed,with 17 groups of underwater topographic survey data and the measured results of fluid mud and silt unit weight in the base trough and slopes.And some results are obtained,including the siltation rate changes and sediment distribution characteristics of the trial dredged-trough,the slope ratio of steady slope,the thickness and change trends of the fluid mud layer and silt layer in the trough.This paper analyzes the different impacts of flood and typhoon on back silting,and points out that the first flood from dry season to flood season will increase siltation remarkably.Besides,based on the statistics of ultrasonic depth-sounding with three different frequencies,it is found that there is a difference of 0.3 m between multi-beam and low frequency,which coincides with the measured values of the fluid mud thickness of 0.29 m very well.