Abstract: As a potential source of groundwater pollution, landfill sites may still cause significant pollution risks to surrounding groundwater due to leachate leakage after closure. Therefore, studying the pollutant transport patterns after leachate leakage in landfill sites is of great significance for managing landfill sites after closure and monitoring groundwater pollution. In this paper, we established a water flow and solute transport model based on Visual Modflow, considering only the abnormal condition of long-term continuous leakage at the bottom of a landfill site, to predict the transport and diffusion laws of typical pollutants (COD) in domestic landfills. The research results indicate that the groundwater flow in the study area flows from north to south, and COD leaks and then migrates downstream; the long-term continuous leakage at the bottom of the landfill area continuously increases the diffusion range, distance, and maximum concentration of the pollution plume. By the 3650th day, it has exceeded the boundary of the research area, causing pollution to the CY River on the right side. Therefore, after discovering the leakage, it is necessary to take effective measures promptly, such as gas phase extraction, PRB technology, etc., to avoid the continued migration and diffusion of pollutants; it is necessary to strengthen the monitoring of groundwater in the landfill and its surrounding areas, regularly inspect and maintain groundwater monitoring wells, and establish a reasonable emergency response mechanism.