Abstract: As a pivotal framework for ensuring food security, optimizing resource utilization, and stabilizing farmers’ income, cropping structure adjustment faces dual challenges of accelerating farmland “de-grainization” and smallholder transformation constraints. Digital financial inclusion (DFI) with its capacity to optimize agricultural resource allocation and alleviate financial constraints emerges as a critical tool for agricultural modernization. Based on the 2018 China Labor-force Dynamics Survey (CLDS) data and applying the OLS regression and the KHB mediation models, this paper investigated the DFI's impacts and mechanism on cropping structure adjustment. Key findings include: 1) There is a significant non-linear “U-shaped” relationship between the DFI development and the cropping structure adjustment, with an inflection point at the DFI index 316.8, demonstrating a sequential “de-grainization” orientation below the threshold and a “grainization” transition beyond it; 2) Heterogeneity analysis reveals the divergent patterns: a classic U-curve persistence in main grain-producing/selling regions versus an inverted U-curve in production-marketing balanced areas due to the terrain constraints, while middle-high income agricultural households exhibit significant responsiveness compared to policy inefficacy among low-income groups constrained by the cognitive and endowment limitations; and 3) Agricultural machinery services mediate 8.7% of the U-shaped relationship through mechanization substitution and scale operation pathways. Based on the empirical findings, this study proposes the following policy recommendations: implementing differentiated policy delivery tailored to regional characteristics, differentiating development pathways categorized by functional zoning, synergistically empowering agricultural machinery services and financial resources, and enhancing the digital-financial infrastructure capabilities. These measures aim to curb the structural shift toward non-grain crop cultivation and safeguard national food security through coordinated institutional innovations in production factor allocation, technological adaptation, and policy implementation.