Abstract: The development of agricultural socialized services is a crucial component in promoting the green transformation of grain production, and it represents an inevitable strategic choice to consolidate the foundation of grain security under the current circumstances. With increasing environmental constraints and the urgent need to improve production efficiency, agricultural socialized services have become an essential mechanism for modernizing agriculture and achieving sustainable development in the grain sector. In this paper, we used the super-efficiency Slack-Based Measure (SBM) model and the global Malmquist-Luenberger (GML) index to measure the grain green total factor productivity of 30 provinces (autonomous regions and municipalities, excluding Xizang, Hong Kong, Macao, and Taiwan) in China from 2002 to 2022, and constructed a dynamic spatial Durbin model to examine the spatial effect of agricultural socialized service development level on the grain green total factor productivity and its regional heterogeneity. The results are summarized as follows. 1) From 2003 to 2022, China’s provincial grain green total factor productivity showed a continuous upward trend and a spatial pattern of being high in eastern China and low in central and western China. 2) China’s grain green total factor productivity and the agricultural socialized service development level both had an obvious positive spatial correlation. 3) In the short term, the improvement in the development level of agricultural socialized services not only enhanced the grain green total factor productivity in the local region but also showed a significant positive spatial spillover effect. This result remained robust across a series of tests. 4) The effect of agricultural socialized service development level was characterized by regional heterogeneity. In the short term, the development level of agricultural socialized services had a significant positive spatial spillover effect on the grain green total factor productivity in eastern China, the main grain-producing region, and the main grain-consuming region; in the long term, only the main grain-producing region with a solid agricultural foundation can continue to give play to the promoting role of agricultural socialized services and drive the neighboring areas to improve the grain green total factor productivity. 5) The level of government intervention, agricultural cropping structure, urbanization level, scale of grain farming per capita, and industrialization level all had significantly positive effects on the grain green total factor productivity in neighboring regions. According to these findings, it is recommended that China strengthen the coordinated governance of agricultural socialized services and green grain production transformation. Furthermore, promoting inter-regional collaboration in agricultural services and optimizing the spatial allocation of green grain production according to local conditions will be essential to advancing the sustainable development of China’s grain economy.