Abstract: The abnormal siltation was present in the E15 foundation trench of the immersed tube tunnel of the Hong Kong-Zhuhai-Macao Bridge,and the installation of the immersed pipe of the submarine tunnel was forced to paused. In order to find out the cause of abnormal silting at the E15 foundation trench,various technical means such as field hydrological and sediment observation,remote sensing image data analysis and numerical simulation have been carried out. In this paper,the water-sand mathematical model is adopted to simulate the diffusion and transport process of the muddy water mass formed by sand mining activities in the upper reaches of Neilingding Island under the action of tidal current,in which the sand source is simulated in the form of surface source. The simulation results show that the muddy water mass formed by sand mining activities can be transported and spread to the foundation trench during the ebb tide of a spring tide,and the daily silt thickness of the E15-E27 foundation trench will be increased by 43.8% under the influence of sand mining activities. The results of numerical simulation provide evidence for finding out the sediment source of abnormal silting in E15 foundation trench. The water and sediment environment in the east of E15 pipe section is complicated. The natural siltation is close to the critical siltation allowable value. The sand mining activity in the upstream water of the foundation trench will bring uncontrollable factors to the siltation at the foundation trench. Therefore,the attention should be paid to the influence of sand mining activities on sediment deposition in surrounding water area.