Abstract: To rationally allocate resources, improve grain production efficiency, and promote the sustainable development of agricultural production, this study utilized panel data from nine provinces in the Yellow River Basin covering the period 2012 to 2023. A data envelopment analysis model was employed to measure grain production efficiency, and models incorporating two-way fixed effects, threshold effects, and moderating effects were used to empirically examine the impact of rural population structure on grain production efficiency and its underlying mechanisms. The study found that an increase in the rural child-to-adult dependency ratio had a significant positive impact on grain production efficiency, whereas an increase in the proportion of the workforce employed in the primary sector exerted a negative inhibitory effect; this conclusion remained valid after robustness and endogeneity tests. The average household size in rural areas exhibited a significant threshold effect in these relationships, meaning that the marginal effects of the rural child dependency ratio and the proportion of the workforce in the primary sector on grain production efficiency differed structurally across different household sizes; socialized agricultural machinery services played a moderating role in the process by which rural population structure influences grain production efficiency. Based on these findings, it is recommended to strengthen the development of human capital among the rural labor force, support business models suited to household size, and optimize the supply of agricultural machinery services in a manner tailored to local conditions.