As the smallest livelihood unit in rural society, farmers are facing the impact of multiple stressors, which not only aggravate the livelihood vulnerability of farmers in key ecological function area, but also threaten the improvement of ecological service function in this area. At present, it is urgent to accurately assess the livelihood vulnerability of farmers under multiple stressors, so as to provide reference for the formulation of sustainable livelihood policies. Taking the Yellow River water supply area of Gannan as an example and using the data of households survey, this paper evaluates the livelihood vulnerability of the farmers under the multiple stressors and analyzes its forming process. The results showed that: (1) livelihood stressors such as family illness, high tuition fees and natural disasters had the greatest impacts on the livelihood of farmers in the Yellow River water supply area of Gannan. Economic stressor was the most frequent stressor faced by the farmers, but natural stressor had the greatest impact on the livelihood vulnerability of farmers in this area. (2) The livelihood sensitivity of farmers in economic demonstration areas was the highest, and the adaptation of farmers in key protection areas was the lowest. (3) Farmers in key protection areas had the highest livelihood vulnerability, followed by economic demonstration areas, and the lowest in the rehabilitation treatment areas. Under the stressor of natural stressor, the farmers had the highest livelihood vulnerability. (4) Different types of livelihood stressors had different paths affecting farmers' livelihood vulnerability. The natural stressor affected livelihood vulnerability by affecting natural resource dependence and natural capital, social stressor through affecting drinking water conditions and social capital, and economic stressor by affecting family dependency ratio and financial capital. The interaction between livelihood stressors increased the livelihood vulnerability of farmers.