Abstract: When encountering water rich sand layers during underground engineering construction in coastal and riverside areas, it is very easy to cause sudden water permeability, mud gushing and sand gushing,causing accidents such as ground collapse, tunnel burial, foundation pit collapse. Taking the water leakage accident in a shield tunnel section of a certain urban rail transit as an example, based on the collection of engineering accident data, combined with the comprehensive analysis of regional geological background, a large number of drilling data, field exploration, field testing and indoor testing results, study the action mode of various important geological factors under the background of the accident,and identify the main control geological factors causing the accident. The geological background of study area is complex, the Quaternary sediments are widely developed and unevenly distributed, with complex and diverse sedimentary products. The groundwater level is high and the groundwater quantity is rich. The groundwater covering with impermeable layers such as silt and muddy soil are mostly under pressure. The stability of composite formation under high water pressure is poor, and the continuous seepage erosion is prone to generating water inrush channels leading to instability of the geological structure. The volcanic rock joints and fissures of the Huachong Formation drilled in the basement are developed, with weak to strong water permeability, medium to good water abundance, and large changes in water abundance and permeability. The development of bedrock water is heterogeneous, and the impact of different lithology and weathering degree on construction varies. The ancient river channels exposed underground are overlapped with the current river network development. The main river channel of Dongping Waterway shows high stability and inheritance. There are medium-strong permeable sand gravel and pebble layers distributed along the ancient river channel. Most areas between the deep confined water and bedrock water do not have an water-resisting layer, and the part of deep confined water is connected with surface water. The exchange of various types of water is smooth. There are various ways of groundwater recharge, and the hydrogeological conditions are complex. The above are the unfavorable geological background factors that are easy to cause water gushing and sand gushing in the water-rich sand layer, especially the formation and distribution of ancient river channels and the development of composite strata sedimentation, which need to be focused on prevention and control in the subsequent underground engineering construction with similar geological conditions.