Urbanization and ecosystems interact and constrain each other, and the balanced relationship between the two is important for the sustainable development of the region. Most existing studies focus on the static coupling relationship between them, and there are fewer studies on the relative development relationship between them. Combining the static coupling coordination degree and the dynamic decoupling model can effectively reveal the dialectical relationship between urbanization and ecosystem services. This study aims to provide an effective scientific reference for the coordinated development of the Loess Plateau. Therefore, this paper took the counties on the Loess Plateau as the study area and measured the urbanization level and the ecosystem service value on the Loess Plateau from 1990 to 2020. With the help of the coupling coordination degree model and decoupling model, the synergistic and relative development relationship between them and the evolution of the process were analyzed. The results showed that the level of urbanization on the Loess Plateau increased steadily from 1990 to 2020. There were obvious spatial differences, showing a spatial distribution pattern of high in the east and low in the west, and expanding to the periphery with the provincial capital city as the center. Over the past 30 years, the total ecosystem service value of the Loess Plateau has been increasing, and the ecosystem service value provided by different land use types was grassland > forest land > arable land > water land > unused land > wetland, and more than 80% of the ecosystem service value was provided by grassland and forest land. The coupling coordination degree between urbanization and ecosystem service value on the Loess Plateau has been increasing, and the proportion of counties in coordination increased from 26.27% in 1990 to 59.70% in 2020, but it was still at a low level. More than 60% of the counties on the Loess Plateau exhibited strong decoupling and weak decoupling, which indicated that the ecosystem services value has not decreased while urbanization has developed. However, the decoupling relationship between urbanization and the ecosystem service value is a dynamic process. The number of counties in strong negative decoupling has been on the rise in recent years and urbanization has increased the potential threat to habitats. All regions of the Loess Plateau should follow a green and high-quality development path based on the differences in natural and socio-economic conditions, avoiding large-scale re-coupling and ensuring the stability and sustainability of the decoupling relationship between urbanization and ecosystem services.