A series of pilot experiments of runoff pollution purification were conducted on riparian buffer strips covered with Cynodon dactylon at different slopes of 2, 3, 4, and 5%. Results showed that there is a significant relationship between slope gradient and intercept rate of suspended solid (SS) carried by runoff. 84% of SS was trapped on the 19m long strip with the slope of 2%, while 70% of SS removal on the slope of 5%. TN and TP removal rates were higher at the slope of 2% than that at 5%, in which average concentrations of seepage at the slope of 2% were lowed up to 1.63 mg N·L-1 and 0.1 P mg·L-1 respectively, while effluent TPs in all experiment treatments were met surface water criteria Class IV (GB3838-2002) with influent TPs varied from 0.9-1.0 mg·L-1. Results also showed that the optimal strip width was 16.1m at the slope of 2%, compared to 24.7m at 5%.