Clarifying the temporal and spatial changes in the trade-off and synergy between ecosystem services and its influencing factors in the Loess Plateau (LP) provides the basis for a better understanding of the interaction of ecosystem services in the background of energy and economic development. It also provides a theoretical basis for the coordinated development of economy and ecological protection. Using the partial correlation spatial statistical analysis, we investigated the spatio-temporal variation in the trade-off and synergy between ecosystem services (net primary productivity (NPP), soil conservation, water conservation, and food supply) and analyzed its influencing factors through the study of Grain-for-Green Program (GFGP) and non-Grain-for-Green Program (non-GFGP). We also assessed the effect of terrain slope on the dynamic changes in the trade-off and synergy during 1993-2013. The results obtained were as follows. (1) Various kinds of ecosystem services in Yanan have different correlations. We found that food supply and soil conservation, food supply and water conservation, and food supply and NPP have a trade-off relationship. There was a synergistic relationship between NPP and soil conservation, NPP and water conservation, and soil conservation and water conservation. (2) In addition, between 1993 and 2013, this synergistic relationship showed a downward trend, whereas, the trade-off relationship demonstrated an upward trend. (3) Furthermore, a comparison of the dynamic changes in the trade-off and synergy between GFGP and non-GFGP showed that various ecosystem services in GFGP have a synergistic relationship, and it continued to increase, but it showed a conflicting relationship in non-GFGP. (4) The conflict between ecosystem services in the area of 15°-25° was obvious, but in the area of > 25°, the synergistic relationship was evident. (5) In terms of the synergistic relationship between NPP and water conservation, the strongest evidence was found in the north of Yanan and the lowest was observed in the south. Moreover, the strongest trade-off relationship between NPP and food supply was in the north, whereas, the weakest was in the east. While exploring the trade-off between water conservation and food supply, we found that it is the strongest in the southest of Yanan and the lowest in the north.