Abstract: The integrated development of rural industries is an important pathway for promoting rural industrial revitalization and sustaining farmers’ income growth. Based on county-level panel data from China for the period 2013-2023, this study treats the establishment of national demonstration parks for the integrated development of rural industries as a quasi-natural experiment and employs a multi-period difference-in-differences model to examine its effects on farmers’ income and the underlying mechanisms. The results show that, first, the establishment of demonstration parks significantly increases farmers’ income, and this finding remains robust after parallel trend tests, placebo tests, heterogeneous treatment effect tests, instrumental variable estimation, and re-examination using double machine learning. Second, the mechanism analysis shows that demonstration park construction promotes farmers’ income growth mainly by improving agricultural production efficiency, extending the agricultural industrial chain, and facilitating the development of rural e-commerce industrial chains. Third, the heterogeneity analysis indicates that the income-enhancing effect of demonstration park construction is more pronounced in industry-oriented counties, and that a higher endowment of distinctive agricultural resources further strengthens the income-enhancing policy effect. Accordingly, future demonstration park construction should be advanced in a differentiated manner according to county-level industrial structures and distinctive agricultural resource endowments. Efforts should also be made to further optimize the internal division of labor in agriculture, improve supporting systems for industrial chains, and enhance the development of rural e-commerce, so as to more fully release the income-enhancing effect of the integrated development of rural industries.