Abstract: In view of the poor nitrogen and phosphorus removal efficiency of the traditional bioretention tank, a study was carried out on the leaching test of immobilized bacteria algae packing, and the nitrogen and phosphorus removal efficiency of biological retention ponds with different proportions of immobilized bacteria algae packing. The characteristics of nutrient release were studied by continuous leaching of immobilized bacteria algae packing in deionized water. G1 group accounting for 2/5 of the filler layer and G2 group accounting for 4/5 of the filler layer were set up to study and analyze the nitrogen and phosphorus removal effects under different submergence heights (0, 30, 60 cm) and drying periods. The results showed that total phosphorus (TP) and total nitrogen (TN) were not detected in the first 8 times leaching of immobilized bacteria algae packing, and immobilized bacteria algae packing was suitable as the improver of bioretention tank packing. With the increase of submergence height, the removal rate of ammonia nitrogen (NH₃-N) and TN by bioretention tank increased. At the submergence height 60 cm, the average removal rates of NH₃-N in G1 and G2 groups were 68.25% and 72.00%, and the average removal rates of TN were 64.20% and 68.70%, respectively. At the submergence height 0 cm or 60 cm, the removal rates of TP in G1 and G2 groups were 79.50% and 78.00%, 70.05% and 71.00%, respectively. At the submergence height 30 cm, the removal rate of TP in G2 group was the highest, reaching 86.00%. When the drying period was extended from 2 days to 8 days, the removal rate of NH₃-N and TN decreased from the highest of 69.38% and 67.10% to the lowest of 55.13% and 57.70%, respectively, while the removal rates of TP increased from the lowest of 75.50% to 90.00%. The test results showed that the immobilized bacteria and algae filler could effectively improve the nitrogen and phosphorus removal performance of bioretention tank.