The water quality of Dianshan Lake in Shanghai Municipality, China, is impacted by nutrient losses from agricultural lands around the lake. In this study, nine types of agricultural land use were monitored in 2010 and 2011, and a correlation analysis between nutrient losses from agricultural non-point sources (NPS) and nutrient stocks in the lake was conducted over monthly and seasonal time periods. The results indicate that the monthly average concentration of total nitrogen (TN) ranged from 1.41 to 7.34 mg/L in 2010 and from 1.52 to 5.90 mg/L in 2011, while the monthly average concentration of total phosphorous (TP) ranged from 0.11 to 0.26 mg/L in 2010 and from 0.13 to 0.30 mg/L in 2011. The annual loss of TN from agricultural NPS was 195.55 tons in 2010 and 208.40 tons in 2011. The cultivation of water oat made the largest contribution to the loss of TN. The annual loss of TP was 44.58 tons in 2010 and 48.12 tons in 2011, and multi-vegetable cultivation made the largest contribution to the loss of TP. The results of correlation analysis show that the monthly stocks of TN and TP in the lake have a positive correlation with the monthly losses of TN and TP from agricultural NPS. According to the seasonal data, the stocks of TN and TP in the lake both have a much stronger correlation with the losses of TN and TP from agricultural NPS in summer than in other seasons. Agricultural NPS pollution control should be the main focus for the water resource conservation in this area.