Vegetation restoration on the Loess Plateau affects the improvement of ecosystem services. Based on the National Field Observation and Research Station of Farmland Ecosystem in Ansai, Shaanxi Province, the two typical small watersheds (Zhifanggou and Fangta) were selected. We used rainfall data, remote sensing interpretation data, social and economic data from 1998 to 2018 to assess four ecosystem service (including soil conservation, carbon storage, water conservation and habitat quality) and to explore the tradeoff and synergy relationships of these ecosystem service. The results showed that vegetation restoration promoted ecosystem services functions (soil conservation, carbon storage, water yield and habitat quality) in the two typical small watersheds, with a slight increase from 2008 to 2018, but a large increase from 1998 to 2008. The forestland and grassland were largely distributed during the initial 10 years from 1998 to 2008, while farmland was largely distributed in the subsequent 10 years from 2008 to 2018. In addition, there was a synergistic and tradeoff relationships of ecosystem services among the two typical small watersheds, which was dominant with the synergistic relationship, due to the influence of natural factors and human factors. Concretely, in 1998, ecosystem service functions exhibited synergy in Zhifanggou, and exhibited tradeoff in 2008 and 2018. In Fangta, ecosystem service functions exhibited tradeoff in 1998 and 2018, while exhibited synergy in 2008. There was a difference in ecosystem service function in different land use types and different areas. In total, water yield, carbon storage and soil conservation showed the synergy relationship. The main reason was that ecosystem services increased differently in time and space in the process of vegetation restoration. Thus, it is necessary to divide and develop appropriate protection strategies according to their trade-offs and synergies relationships among ecosystem service functions.