Abstract: Based on panel data from four provinces (cities) in Southwest China from 2013 to 2022, this study empirically investigated the impact of the transformation of farmland utilization function on food security level in Southwest China, and further explored the threshold effect of farmland utilization function transformation on food security under the background of “Non-grain cropland” using a threshold effect model. Research results showed that: (1) The level of food security in the Southwest region was evolving towards a stable and positive trend, with a fluctuating decline in the early stage of the transformation of farmland use function, and a steady increase in the later stage. The rapid development of the national economic contribution function had played a core driving role in the transformation of farmland utilization function. (2) The functional transformation of farmland utilization in the southwest region significantly improved the level of food security, and there was a threshold effect. When the level of “Non-grain cropland” reached a certain value, its promoting effect was weaken. (3) The results of heterogeneity showed that in Yunnan and Sichuan regions, as well as areas with significant financial support for agriculture, the promotion effect of the transformation of farmland utilization function was also more prominent. Therefore, it is essential to implement differentiated regional policies, prioritize the economic contribution of cultivated land to the national economy, improve the agricultural support and protection system, and remain vigilant against the risk of “Non-grain cropland” during the process of the transformation of farmland utilization function, so as to prevent it from undermining food security.