By monitoring the water purification effect of the farmland drainage wetland in the remediation of Gushi Bridge River in Wuxi on the west bank of the Wangyu River, the removal effect of nitrogen, phosphorus and organic pesticides in the paddy field backwater by the farmland wetland was studied and the removal mechanism was analyzed. The results show that the average removal rates of total nitrogen, total phosphorus and NH₃-N by the farmland wetland were 60.95%, 72.62% and 65.97%, respectively. Removal rates of three typical pesticides including MCPA-Na, Triazophos and Carbendazim ranged from 20.86% to 62.33%, 17.86% to 69.52% and 1. 1% to 61.15%, respectively. Nitrogen and phosphorus in the water body could be removed through physical interception and filtration of the permeable dams, substrate adsorption, microbial metabolism as well as plant absorption and utilization. At the same time, it was found that the removal rates of pollutants by the farmland wetland were closely related to the filler media used in permeable dams at all levels. There were no significant differences in the removal of pollutants by wetland pools at all levels. The pesticides in the water body were removed in a single way, which mainly depended on the effect of functional microorganism on the surface of the filler, and the removal rate of pesticides by the first-stage permeable dam was significantly lower than that of the last two levels.