Abstract: Grain security is a top priority in governing the country. Currently, global temperatures have exceeded critical thresholds, posing a severe threat to the stability of grain output. Based on panel data from 221 prefecture-level cities in China from 2013 to 2022, this study constructs a fixed-effects model using a production function, introduces an "interaction term between topographic slope and year" as an instrumental variable for capital input to mitigate endogeneity, and ensures the reliability of conclusions through multi-dimensional robustness tests. The empirical results show that extreme climate has a significant negative impact on grain output. Heterogeneity analysis indicates that major production areas are significantly negatively affected by extreme high temperatures; production-consumption balance areas suffer from the dual significant impact of extreme high temperatures and heavy rainfall; while major consumption areas are sensitive to extreme drought. Mediation effect tests reveal that extreme climate has a significant negative impact on capital input, forming an indirect path of "climate shock-capital inhibition-output decline". Moderation effect analysis shows that capital input can significantly mitigate the impact of extreme climate, and the synergy between labor and capital can further strengthen this moderating effect. This study advances existing research in terms of data granularity, identification strategy, and regional perspective, and proposes a policy mix encompassing regional disaster prevention, capital optimization, factor synergy, and institutional guarantees. It provides operable paths for enhancing agricultural climate resilience and the stability of grain output.