Abstract: Ecological ponds are the end link of the control technology system of agricultural non-point source pollution in the water network area of southern China. They should be constructed according to local conditions to effectively intercept nitrogen and phosphorus pollutants in farmland runoff, thereby minimizing the emission load of non-point source pollution, which is significant for the improvement of rural water environment. Based on the rice-oilseed rape cropping system, the ecological transformation was carried out at the end of the small watershed hydrological pathway, and the ecological branch and advanced purification pond were constructed. Four sampling sites were set along the direction of the water flow in the ecological pond. Annual water samplings at these sites were carried out and the concentrations of TN, \mathrmNH_4^+ -N, TP, and COD were monitored. The annual purification efficiency of the ecological pond was analyzed, and the configuration scale of the ecological pond corresponding to unit farmland area was discussed. The results showed the average total removal rates of TN, \mathrmNH_4^+ -N, TP and COD in the ecological pond were 40.1%, 47.7%, 27.3% and 38.1%, respectively. In the rice season, the total removal rates of TN, TP and COD along the ecological pond were higher than those in the oilseed rape season. Under low temperature conditions in winter, the concentrations of TN, TP, and COD at the outlet of the purification pond were still lower than those at the inlet of it. To estimate the configuration scale of the ecological pond based on the loss of nitrogen and phosphorus from farmland and the removal efficiency of the ecological pond, the area ratio of farmland to ecological pond is approximately from 23∶1 to 29∶1. To estimate the configuration scale of ecological pond based on runoff volume from farmland, the area ratio of farmland to ecological pond is approximately from 21∶1 to 33∶1. Further research should be conducted on measures of storing muddy water, releasing clear water and storage volume of initial runoff, to decrease the configuration scale of ecological ponds corresponding to unit farmland area.