The interaction between agricultural and pastoral cultures during the Bronze Age has attracted widespread attention. However, these exchanges remain unclear in the frontier zone where pastoral and farming cultures intersected, particularly within China's Northern Agro-Pastoral Ecotone. Here, systematic stable carbon and nitrogen isotope analyses were conducted on crops, animal bones, and human bones excavated from the Bronze Age Shuiquan site in the eastern ecotone. We also integrate published zooarchaeological, archaeobotanical, and stable-isotope data from three different sub-regions of this transition zone to explore the interaction between introduced livestock and local millet agriculture-specifically, the dietary contribution of introduced animals to humans and the influence of local millet farming on livestock management. Our results indicate that: (1) at the Bronze Age Shuiquan site, in the Eastern part of the ecotone, humans primarily consumed millet crops (52 % +/- 5 %), including foxtail and common millet. Animal protein was derived mainly from domestic omnivores, especially pigs and probably dogs (34.6 % +/- 4.8 %), whereas introduced herbivores such as sheep and cattle contributed much less (10.8 % +/- 3.1 %). In addition to provisioning pigs and dogs with millet, local inhabitants also kept cattle and a small number of sheep, feeding them millet by-products such as straw or chaff. (2) Regional comparisons across three sectors of the ecotone-Liaoxi, Hetao, and the Western Loess Plateau-show that millet remained the staple dietary source. However, in Hetao and the Western Loess Plateau, introduced herbivores (cattle, sheep) surpassed local omnivores (pigs, dogs) as the primary source of animal protein. Moreover, the contribution of millet to the diets of cattle and sheep exhibited a clear east-to-west decline. (3) Diet and subsistence diversity across the three regions reflects variations in geographical environment, resource reserve, and population migration from various cultures. These findings illustrate the range of subsistence strategies adopted within the Northern Agro-Pastoral Ecotone during the Bronze Age, offering scientific evidence to advance our understanding of the interaction of agro-pastoral interactions in prehistoric Eurasia.