Abstract: Surplus phosphorus is a main source of non-point source pollution. Tuojia, a small agricultural watershed (52 km²) and Jianshan, a small forest-agricultural watershed (50 km²), located in Changsha County, Hunan Province, are taken as the case study. Based on the household surveys and 4-successive-year of observations of the phosphorus (P) flows at the terminal of river, substance flow analysis was used to quantify the budgets of P. Results showed that the concentrations of P in the agricultural ranged 0.03-0.68 mg·L^-1, with the average 0.21 mg·L^-1, while the corresponding one in the forest-agricultural ranged 0.01-0.35 mg·L^-1 with the average 0.08 mg·L^-1. The surface water quality in the forest-agricultural watershed was obviously better than that in the agricultural watershed. The phosphorus surplus intensity in the agricultural watershed was apparently higher (32.0 kg·hm^-2·a^-1) than that in the forest-agricul-tural watershed (20.6 kg·hm^-2·a^-1), and thus had a greater effect on environment. In 2013, livestock feeding accounted for 53.1% of total P inputs in the agricultural watershed, while fertilizer contributed 53.0% in the forest-agricultural. Generally, the phosphorus from plant and animal products dominated in both watersheds, approximately accounting for 94.0% of the total. Thus, controlling the amount of P fertilizer and improving its utilization efficiency are good ways to mitigate the eutrophication in the study area.