The purpose of this study was to calculate a phosphorus (P) budget for the intensive agriculture practiced Haean watershed located in Yanggu county, Gangwon province, South Korea. Moreover, a mass-balance approach used to develop the P budget, which linked P inputs in arable lands concealed by intensive agriculture practices to P outputs both field and watershed scale. Fertilizer (chemical and compost) and atmospheric deposition considered as P inputs. Whereas, outputs included crop harvesting and hydrologic export to Mandae stream. Initially, the total P input to the watershed (302,132 kgP/yr) and total output (206,906 kgP/yr). Overall, fertilizers were the biggest sources of P input in the watershed (99%) and the biggest outputs were P in harvested crops (83%). In addition, hydrologic export of P accounted for (17%) of the total output. These results elucidate that the P budget of the Haean watershed is positive, showing that (32%) of P surplus in the watershed. Consequently, this P surplus might be a potential source of nonpoint source pollution to the water bodies of the area, suggesting that balanced fertilizer application based on soil P requirement might be effective for the control and management of agricultural nonpoint source pollution.