Quantitative evaluation of soil conservation effects (SCE) of ecological policies at the macro level is not only a summary of past ecological recovery experience but also an effective way to explore the optimization of future ecological policies. As a famous ecological policy, “Grain for Green” (GFG) carried out in the Loess Plateau of China has significantly reduced soil erosion in this region. However, it is unclear whether the implementation of this policy has achieved the optimal SCE, which is an important issue in the GFG evaluation. The concept of soil conservation potential (SCP) was proposed in this study. Taking Yan’an, China as an example, the influence of GFG on soil erosion control was evaluated by comparing the total amount of soil erosion reduction and the SCP under certain the GFG intensity. On this basis, the future measures of GFG optimization were proposed. The result shows that the implementation of GFG in the past does not take into account the local SCP, and there is a significant spatial mismatch between the GFG implementation intensity and the SCP, resulting in the waste of resources and low policy efficiency. If this problem was fully considered in ecological restoration planning, a “win-win” result of improving governance effect and reducing input cost could be achieved. Finally, the suggestion of establishing the GFG dynamic input mechanism was put forward for the formulation and optimization of relevant ecological policies in the future.