Baiyangdian watershed is one of the important freshwater sources in the Xiong'an New Area. To protect the ecological environment, a series of ecological restoration projects have been implemented since the 1980s. The projects improved the ecological environment but with the reduction in farmland area as cost. Therefore, this study tried to find a better way to optimize the rules of the farmland conversion to reduce trade-off between the improvement of water quality purification and the decrease of farmland area by setting up different scenarios of farmland conversion. The results show that: In the condition that converts farmland with the same area, the efficiency of returning farmland in the riparian zone is higher than the slope field; In the condition that converts farmland under the same rule, the water quality purification efficiency of returning farmland into forest is higher than returning farmland into shrubland or grassland. The improvement rate of per unit water quality purification is reduced with the increase of buffer distance in the riparian zone. Under the scenario that returning farmland into forest in 300 m riparian zone, the improvement of water quality purification rate is higher than that of the actual scenario (2015), but the reduction in farmland area is lower than that of the actual scenario (2015). By optimizing the rules of returning farmland, the pattern of ecosystem restoration can be appropriately optimized. And it is not only conducive to improve the target ecosystem services, but also reduced trade-off between different ecosystem services improvement goals.