Agricultural diffuse pollution caused by runoff and soil loss from cropland has been a serious environmental problem in the black soil area of Northeast China. While water and soil conservation measures have led to a sharp decrease in overland flow within source water sub-watersheds, they have, on the other end, affected the water security of agricultural and domestic use as the water storage capacity of downstream reservoirs decreases year by year. For this study, grass barriers, which are generally considered to have high sediment reduction efficiency, were evaluated for their efficiency as a mitigation measure of diffuse pollution within source water areas. Standard field plots were used to monitor two groups of grass barriers, alfalfa (Medicago sativaL.) and native grass (dominated by Setaria viridis(L.)Beauv.), and slope gradients (3 degrees and 5 degrees) under natural rainfall events over the 2012-2015 period. The results indicated that, during the maize (Zea maysL.) growing season, runoff and sediment reduction efficiencies for the alfalfa barrier were 37.91 % and 62.30 %, respectively. And more than 50 % of the nutrient loads (e.g., total nitrogen (TN), total phosphorus (TP), and nitrate (NO3--N)) could be removed by the alfalfa barrier. The environmental loads mitigation efficiency of grass barrier in spring was relatively low and unstable. Both slope and barrier width had a significant impact on the water and soil conservation function of grass barriers, while rainfall intensity and grass type were mainly related to runoff reduction. Overall, these finding could provide some specific recommendations for future grass barrier design and large-scale application in a source water area of Northeast China.