Abstract: The agro-pastoral ecotone in North China is a typical ecologically fragile zone. Scientific and reasonable definitions are important for the sustainable development of regions based on resource utilization. The water conditions are the most basic conditions to define the agro-pastoral ecotone in North China. Previous studies used precipitation as the water condition and did not consider the impact of irrigation. Based on the traditional method, this study incorporated irrigation corrections and used the corrected water condition, combined with the water variability and dryness indicators and with support of the fuzzy comprehensive evaluation method, to define the agro-pastoral ecotone in North China. Subsequently, this study also combined the administrative boundaries to divide the counties in the agro-pastoral ecotone. The results show that the agro-pastoral ecotone in North China presents a banded distribution in the northeast-southwest direction, with a total area of 659 000 km². There are 123 counties (cities, banners) in the agro-pastoral ecotone, with a total area of 660 800 km², distributed across nine provinces (autonomous regions) in North China. The number of counties is highest in the Inner Mongolia Autonomous Region, Shanxi Province, and Qinghai Province, and the total area of the counties is the largest in the Inner Mongolia Autonomous Region. The results of this study shift slightly northwest of the range designated by the Ministry of Agriculture and extend northeast and southwest, respectively, in scope. Three provinces, Heilongjiang, Jilin, and Qinghai, were added, while Liaoning Province was removed. There has been a decrease in the total number of counties. The total number of counties (cities, banners) in the agro-pastoral ecotone in Heilongjiang, Jilin, Inner Mongolia, and Qinghai increased, while the total number of counties (cities) in the agro-pastoral ecotone in Hebei, Shanxi, Shaanxi, Gansu, and Ningxia decreased. Some counties in the Inner Mongolia Hetao Irrigation District and Ningxia Hetao Irrigation District were included. Analysis of the hydrothermal conditions in the agro-pastoral ecotone show that the annual accumulated temperature in most areas is between 2000-3500 ℃, with high temperatures in the middle and low temperatures in the northeast and southwest. The water conditions (precipitation and irrigation) in most areas of the agro-pastoral ecotone are in the range of 300-450 mm, decreasing in the southeast-northwest direction. The content of topsoil organic carbon in most areas is between 0 and 1%, with low values in the middle and high values in the northeast and southwest. This study incorporates irrigation indicators into the definition index system of the agro-pastoral ecotone, which compensates for the shortcomings of the traditional climate definition method and is a scientific supplement to the definition of the agro-pastoral ecotone. The obtained county-scale agro-pastoral ecotone can provide a scientific reference to adjust the agricultural-pastoral structure for precise sustainable development of the agro-pastoral ecotone if combined with the hydrothermal conditions.