Adaptation provides a new perspective for the study of farmers’ livelihood transformation and sustainability. This research aimed to explore the influencing factors of different types and adaptation of farmers’ livelihood transformation during rural transformation. Based on summarizing the response of farmers’ livelihood transformation during the rural transformation process in the Qinling Mountains of China from 1990 to 2018, this research constructed an evaluation index system for the adaptation of farmers’ livelihood transformation. Through questionnaires and interview surveys and based on the classification of farmers’ adaptive behavior, the research measured the adaptation index of different types of farmers’ livelihood transformation during different rural transformation periods and analyzed the factors affecting the types and the adaptation of farmers’ livelihood transformation. The results showed that: ① From 1990 to 2018, the livelihood transformation of farmers in Shangzhou District of Qinling Mountains mainly experienced changes from the farming + the working oriented to the synthetic type + the working oriented. ② From 1990 to 2018, the adaptation index of farmers’ livelihood transformation increased significantly in growth. From 1990 to 2009, the adaptation index of farmers’ livelihood transformation of working oriented and synthetic type was relatively high and concentrated. The distribution of the adaptation index of farmers’ livelihood transformation in 2010–2018 was relatively scattered. ③ Farmers with richer social networks tended to choose comprehensive and diversified livelihoods. Farmers with richer financial and natural capital were more likely to choose non-agricultural livelihoods. Leadership potential and social network, livestock, income status, and actual cultivated area were the key variables that have been influencing the adaptation of farmers’ livelihood transformation in Shangzhou District since 1990. The research results contribute to the optimization of sustainable livelihood strategies for farmers in the Qinling Mountains of China and provide case references for the study of livelihood transformation of farmers in underdeveloped mountainous areas worldwide.