Water inrush and sand disasters often occur during tunnel construction in sandy dolomite formations, which can significantly impact project progress and pose a threat to construction safety. However, the seepage failure mechanism and destructive process of sandy dolomite have not been fully researched. This study utilizes PFC3D particle flow software to create a flow-solid coupling model for sandy dolomite. It outlines the mechanism of seepage failure, compares the risk level of rock mass permeation damage at varying degrees of sandification, and includes a case study from the Dianzhong Water Diversion Project. The results show that sandy dolomite has a loose structure with low strength. The excavation face formed by tunnelling will destroy its original mechanical equilibrium state. Groundwater will pass through voids filled with movable particles and carry them away, leads to an increase in porosity rate, enhanced permeability, wider seepage channels and faster seepage velocity. When particle loss reaches critical value (particle loss mass ratio exceeds 5%), it will cause seepage failure and lead to a sudden water inrush and sand accidents. There are three different stages observed in particle loss process for sandy dolomites with increasing hydraulic gradient (0.1~1). As dolomite becomes sandier, the proportion of fine particles gradually increases along with the percentage of total particle mass loss due to groundwater flow.