Farmland quality protection is an important measure to implement the strategy of"storing grain in the land" and a vital part of promoting ecological agriculture development. This study focuses on the main agents of farmland quality protection, farmers, with a sample of 1,013 households from the rice-growing areas of Jiangxi Province, which is one of the major rice-growing provinces in southern China. An ordered probit regression model is used to investigate the influence and mechanism of social capital on farmers' behavior in protecting the quality of farmland. The result shows that:(1)Social capital significantly boosts the farmers' behaviors of farmland quality protection. The promoting effect of bonding social capital is greater than that of linking social capital. These conclusions remain robust after the endogeneity issue has been addressed and robustness tests have been conducted. (2) Ecological cognition plays a mediating role in this relation, while Internet use exerts a significant positive moderating effect. (3) The effect of social capital is greater for full-time farming households than for part-time farming households, and more significant for risk-neutral and risk-appetite farmers than for risk-averse farmers. Accordingly, this study proposes recommendations, including fostering farmers' social capital, improving their ecological cognition, promoting the penetration and use of the Internet, vigorously cultivating new agricultural business entities, and expanding agricultural insurance coverage.