Abstract: The strategy of agricultural scale expansion serves as a crucial means for achieving high-quality development in China’s agriculture. Based on panel data from 31 provinces from 2011 to 2022, this study employs entropy weighting, two-way fixed-effects models, mediation effect models, and spatial effects of econometric models to examine the impacts, underlying mechanisms, and spatial effects of farmland management scale on high-quality agricultural development in China. Results show that: 1) farmland management scale exhibits an inverted U-shaped relationship with high-quality agricultural development, with an inflection point at 2.83 hectares per household; 2) farmland management scale shows an inverted U-shaped relationship with high-quality agricultural development in both the eastern and western regions, with the inflection point in the eastern region being smaller than that in the western region, while a linear positive relationship is observed in the central region. An inverted U-shaped relationship is also found in both grain-producing and non-grain-producing regions, with the inflection point in grain-producing regions being smaller than that in non-grain-producing regions. Moreover, farmland management scale exhibits an inverted U-shaped relationship with agricultural green development and industrial efficiency, while maintaining a linear positive relationship with production efficiency; 3) the expansion of farmland management scale promotes agricultural mechanization, thereby influencing high-quality agricultural development. It also exerts an inverted U-shaped effect on agricultural resource allocation efficiency, which in turn affects high-quality agricultural development; and 4) farmland management scale generates an inverted U-shaped spatial spillover effect on the high-quality agricultural development level of high-quality agricultural development in neighboring regions. Based on these findings, this paper proposes policy recommendations for promoting high-quality agricultural development from three perspectives: exploring a “small-scale + precision” agricultural development model suited for China, formulating differentiated agricultural development pathways, and continuously guiding positive spillover effects while establishing regional collaborative governance mechanisms.