This study explored a cost-effective biogas slurry cascade treatment system aimed at addressing the challenges of managing large quantities of biogas slurry in China. Using a hydroponic system, the research evaluated the purification efficiency in cascade treatment of three aquatic plants— Allium tuberosum, Oenanthe javanica (Blume) DC., and Lactuca sativa var. ramosa Hort. The results showed that the cascade treatment system effectively removed pollutants from the biogas slurry while significantly enhancing plant growth performance. Specifically, after 70 days of treatment, the system achieved removal rates of 78.0% for total nitrogen (TN), 89.5% for ammonia nitrogen (NH4 +-N), 89.2% for total phosphorus (TP), and a 47% reduction in chemical oxygen demand (COD). Additionally, the system substantially reduced the levels of suspended solids and pathogenic bacteria, resulting in a treated biogas slurry that meets environmental discharge standards and can be efficiently utilized as a valuable resource. This study provides a feasible and environmentally friendly solution for biogas slurry management with significant potential for broader application.