Abstract: In order to study the role of wetland substrate on the purifying agricultural runoff, Substrate of constructed wetland for treatment agricultural runoff was studied. Results show that the substrate in the constructed wetland systems can absorb a lot of pollutants, and there were many microorganisms on the mature substrate surface. Experiment showed that the substrate had a strong ability of biodegradation, and played an important role in the process of pollutants removal.the removal efficiency of chemical oxygen demand(CODcr),total nitrogen(TN), total phosphorus (TP )andammonium nitrogen(NH4+-N) in the constructed wetland substrate were 41%, 50%, 49% and 52%, respectively. After the disposal of the wetland substrate, Effluent COD, TN, TPand NH4+-N were GradeⅡ, Grade Ⅴ, GradeⅤand Grade Ⅲ, respectively.