Abstract: A field experiment was conducted in the Chaohu Lake Region to analyze the characteristics of nitrogen and phosphorus loss via runoff from farmlands under natural rainfall conditions. Two treatments of conventional tillage (CK) and straw mulch (TS) were set up in 3 replications. The results show that straw mulch effectively reduces surface runoff, and sediment and nutrient loss from farmlands under natural rainfall conditions. During maize growth period, the amounts of runoff and sediments in TS decrease by 30.47% and 22.88%, respectively compared with CK. This implies that straw mulch has a significant effect on soil and water conservation. Straw mulch respectively reduces total nitrogen (TN) and total phosphorus (TP) loss via surface runoff by 27.42% and 32.29%. However, straw mulch has an insignificant influence on TN and TP concentration of runoff. While dissolved nitrogen (DN) is predominant form of nitrogen loss, particle phosphorous (PP) is the main form of phosphorus loss. In order to control agriculture non-point source pollution, straw mulch should be strongly promoted in the study area.