Abstract: Trade-off and synergy relationships of farmland ecosystem services is not only important research topic in the agricultural ecosystem but also closely related to cropland protection and food security. However, existing researches lack analysis on trade-off and synergy relationships of farmland ecosystem services in Longdong Loess Plateau, where fragile eco-environment (e.g. severe soil erosion, scarce water resources, etc.) seriously threatens cropland resources and land productivity. In order to clarify trade-off and synergy relationships of farmland ecosystem services, Longdong Loess Plateau, one of the birthplaces of Chinese farming civilization, was selected as the study area, and six ecosystem service models, including precipitation storage, universal soil loss equation, and InVEST etc were utilized to calculate the amount of farmland ecosystem services in 2020 based on the socio-economic and resource-environmental data such as the statistical yearbooks, land use/cover, digital elevation models, normalized difference vegetation index, meteorology, soil, net primary productivity, and road POI. Meanwhile, the Pearson correlation coefficient and the bivariate global/local Moran's index were used to distinguish trade-off and synergy relationships between providing services and other services, and identify the spatial heterogeneity of the relationships among services, respectively. The results showed that: 1) The amount of food provision service, water conservation service, carbon sequestration service, soil retention service in 2020 was 4.61×10⁶ t, 2.45×10⁸ m³, 2.94×10⁶ t, 6.61×10⁹ t, respectively, and the mean value of habitat quality service was 0.26. Both providing services and cultural services shared a distribution pattern of “high in the east and west, low in the central and north”, due to the spatial heterogeneity of climate, soil, terrain and socio-economic development planning. By contrast, both adjusting services and supporting services had a "belt-like" distribution. 2) The correlation coefficient between providing services and adjusting services, supporting services, and cultural services was 0.97, 0.85, and 0.97, respectively. And the correlation coefficient between food provision service and water conservation service, carbon sequestration service, soil retention service, habitat quality service, landscape service was 0.94, 0.99, 0.93, 0.82, and 0.97, respectively. All of correlation coefficients showed that all relationships of farmland ecosystem services were synergistic in Longdong Loess Plateau. 3) Trade-off and synergy relationships among four types of services, and among each sub-service of farmland ecosystem had spatial auto-correlation, and significant spatial heterogeneity. Synergy relationships among each sub-service were clustered obviously, appeared mainly in the central and northern Longdong, while trade-off relationships among each sub-service were distributed sporadically. Therefore, it is very significant to maintain multifunctionality of farmland ecosystem in Longdong Loess Plateau, to implement zoning management strategies, to fully exploit synergistic effect of various services of farmland ecosystem, to integrally improve service functions of farmland ecosystem (especially food provision service), and further consolidate the position of "Longdong Grain Warehouse" in the new era.