The Impact of State-Owned Grain Enterprises’ Purchasing and Selling Behavior on Grain Price Volatility: Evidence from China’s Corn Market

State-owned grain enterprises (SOGEs) in China serve dual roles as implementers of policy-oriented stockpiling and as market-oriented operators. Their purchasing and selling behaviors profoundly influence grain price volatility, which in turn affects farmers’ incomes and the stability of agricultural operations. Based on provincial-level monthly panel data from January 2013 to December 2023, this study constructs price volatility indicators for the grain procurement and wholesale market. A fixed-effects model examines the impact of SOGEs’ purchasing and selling behaviors on grain market price volatility, followed by analyses of mechanisms and heterogeneity. The findings reveal that, during the centralized market arrival period, the purchasing behavior of SOGEs generates a significant demand shock, amplifying price volatility in the grain procurement market. By contrast, their selling behavior exhibits counter-cyclical adjustment characteristics in the wholesale market, mitigating price deviations through inventory regulation and guiding price fluctuations back toward supply-and-demand fundamentals. Mechanism tests indicate that market power, price expectation guidance, and the strategic responses of private enterprises are the primary channels through which SOGEs’ behaviors affect price volatility. Heterogeneity analysis shows that external shocks and regional development disparities significantly influence the market stabilization effects of SOGEs’ purchasing and selling behaviors. This study provides mechanism-level empirical evidence for understanding the price influence of SOGEs in grain markets, offering valuable insights for improving counter-cyclical grain regulation policies.