Water conservation function is one of the most important ecosystem service functions of river basin ecosystems, and it is also the key factor for sustainable development.We used the InVEST model to quantitatively evaluate the water yield and conservation function of the Shiyang River Basin using parameter localization, and graded the importance of water conservation function. Furthermore, we analyzed the differences in the water yield and water conservation function by simulating different farmland conversion scenarios. We obtained the following results:(1)When the Zhang value was 2.1 in the InVEST model, the simulation results of the water yield in the study area were the best. The average water production depth in the study area was 60.90 mm, and the total water yield was 24.71×10⁸ m³.The average water conservation of the unit was 23.97 mm, and the total water conservation was 1.35×10⁸ m³ in 2015. The unit water depth and average spatial distribution of the water conservation unit showed a north-south high-low trend. The unit and total water conservation capacities were the highest for the forest and the grassland,respectively.(2)The water conservation function of the generally important area which refers to poor water conservation function had a proportion of 53.49%,whereas the very important region accounted for 22.40% of the total area.(3)The scenario simulation results of the different returning farmland models revealed that when the forest area was the largest, the water conservation capacity and the total water conservation amount were the highest. Furthermore, increasing the forest land and grassland area decreased the water yield, and when the forest area was the largest, the water yield was the lowest. Based on the minimum ambiguity method, a model of returning farmland to forest and grassland was developed.