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By 4000 BP, trans-Eurasian agricultural exchanges increased across the Hexi Corridor. However, the nature and timing of many early prehistoric agricultural exchanges remain unclear. We present systematically collected archaeobotanical data from the ancient Haizang site (3899–3601 cal a BP) within the Hexi Corridor. Adding to previous archaeobotanical studies of the Hexi Corridor, we find that agricultural production transformed from purely millet-based agriculture during the Machang Period (4300–4000) to predominantly millet-based agriculture increasingly supplemented with wheat and barley during the Xichengyi and Qijia periods (4000–3600 BP). These transformations are likely due to adaption to a cooler and drier climate through cultural exchange. A warm and humid climate during 4300–4000 BP likely promoted millet agriculture, Machang cultural expansion westward, and occupation across the Hexi corridor. However, after the “4.2 ka BP cold event” people adopted wheat and barley from the West to make up for declining millet agricultural productivity. This adoption began first with the Xichengyi culture, and soon spread further eastward within the Hexi Corridor to the Qijia culture.