In recent years, with increasing global warming, extreme weather such as high temperatures and droughts has occurred more frequently, with serious impacts on grain production and farmers’ livelihoods. High temperatures and drought can lead to a range of production problems such as water shortages, increased irrigation costs, reduced land fertility, and reductions in grain yields. Ultimately, these problems can inhibit farmers’ willingness to grow grain and threaten regional food security. The government usually takes appropriate assistance measures to compensate for the loss of grain farmers, so as to stimulate the farmers’ incentive to grow grain. Therefore, the influence mechanism of government assistance on farmers’ willingness to grow grain in the context of extreme weather is an issue worth exploring. From the perspective of grain farmers, this paper takes the grain farmers in Jiangxi Province affected by high temperatures and drought in 2022 as the research sample and empirically analyzes the influence mechanism of government assistance on farmers’ willingness to grow grain. In addition, this paper also discusses the heterogeneity of the impact of government assistance on the planting willingness of farmers with different agricultural income ratios and literacy levels. The Ordered Logistic model and mediation effect model were applied in this study. The results of the study show that government assistance can effectively incentivize farmers’ willingness to grow grain, and risk preference has a mediating effect in the process. In addition, government assistance has a stronger incentive effect on the planting willingness of farmers with a high ratio of agricultural income and a high level of literacy. Finally, the paper puts forward corresponding countermeasure suggestions based on the conclusions to help the government formulate more complete risk response policies and establish a more effective risk prevention mechanism. This paper provides a theoretical reference for achieving sustainable development of grain production in the context of extreme weather, which helps to improve the resilience and sustainability of grain production.