Abstract: Major grain-producing areas bear the double burden of food and ecology security. Therefore, it is practically significant to explore the influence mechanism of agricultural social service on the intensity of agricultural carbon emissions to promote the low-carbon transformation of agriculture in the major grain-producing areas. This study analyzed panel data from 13 provinces (autonomous regions) in major grain-producing areas from 2008 to 2022 using the Ordinary Least Squares Method, the Mediation Effect Model, and Spatial Durbin Model of the time fixed effect to determine the direct and indirect impacts of agricultural social service on agricultural carbon emission intensity and spatial spillover effects. The main conclusions of this study were as follows: 1) Agricultural social service had a significant “carbon reduction effect” on agricultural carbon emission intensity. From the perspective of the four dimensions of agricultural social service, agricultural information services, rural public services, and agricultural finance and insurance services reduced agricultural carbon emission intensity, while agricultural material services increased agricultural carbon emission intensity. 2) Based on their level of economic development, the main grain-producing areas could be divided into three major river basins. It was found that the agricultural social service in the Yangtze and Yellow River basins reduced agricultural carbon emission intensity to a certain extent, while the agricultural social service in the Songhua River Basin increased agricultural carbon emission intensity to a certain extent. 3) The agricultural operation scale and agricultural financial support were found to have mediating effects on agricultural carbon emission intensity; that is, agricultural social service can indirectly reduce agricultural carbon emission intensity by promoting the expansion of agricultural operation scale and reducing the negative environmental effects caused by agricultural financial support. 4) Agricultural social service in the main grain-producing areas had a significant negative spatial spillover effect on the intensity of agricultural carbon emissions, and every 1% increase in agricultural social service in this province reduced the intensity of agricultural carbon emissions in neighboring provinces by 7.035%. Based on these findings, the government should improve the development level of agricultural social service in the provinces of the main grain-producing areas, as well as reasonably plan the development direction and scope of agricultural social service. Further developments should seek to build communication channels between agricultural social service and agricultural carbon emission reduction in the provinces of the main grain-producing areas. This will help to reduce the intensity of agricultural carbon emissions and assist in transforming the main grain-producing areas into green and low-carbon agriculture in the near future.