Abstract: Damage to impermeable structure in landfill sites can lead to leachate leakage, posing serious threats to the safety of the surrounding groundwater environment. To investigate the migration and diffusion of leachate in the groundwater under the impermeable structure damage scenarios, this study employed three-dimensional geological modeling to achieve fine characterization of the impermeable layer. We constructed a numerical model of leakage and migration of the characteristic pollutant ammonia nitrogen, and systematically analyzed the differences in the migration characteristics of ammonia nitrogen among different damaged structures of the impermeable layer of the landfill in Guangxi Autonomous Region. The research results showed that the impermeable layer damage pattern had a controlling influence on the diffusion of pollutants, and the linear crack damage along the slope was prone to form a "tongue-shaped" plume with rapid diffusion and high pollution concentration through concentrated channels along the linear shape. A single large hole damage at the bottom was easy to form a strong tail pollution plume, which had the furthest pollution migration distance, with the maximum migration distance of 188.9 m. The damage of multi-point grid-like seepage group was easy to produce a large area of diffuse pollution surface, which had the widest scope of pollution, with the pollution area of 21 330 m². The simulation results and field measurements suggest that the ammonia nitrogen pollution detected in the downstream groundwater is likely attributable to leachate seepage from a single large hole in the bottom damaged impermeable layer of the studied landfill. This study provides theoretical support for the risk assessment of landfill pollution and the construction of prevention and control system.