Abstract: In the face of the real dilemma that traditional small farmer management cannot meet the sustainable development of agriculture, the reduction of chemical fertilizers is an important measure for China’s agriculture to move towards high-quality development, and it is also the key to the development of green agriculture. Based on the theory of professional division of labor, a theoretical model of “service outsourcing”, a division of labor economy, and the reduction of chemical fertilizer is constructed. Using statistical yearbook data from 2011 to 2020, this study empirically analyzed the affacting mechanism of agricultural socialized services on the reduction of chemical fertilizer by using an intermediary adjustment model and an instrumental variable model. The study found that agricultural socialized services had a significant negative impact on the total use of chemical fertilizer and the use per unit area; that is, for every 1% increase in the output value of agricultural socialized services, the total amount of fertilizer used and the amount of fertilizer used per unit area can be reduced by 0.055% and 0.443%, respectively. The robustness test of the tool variables also confirmed the positive effect of socialized agricultural services on the reduction of chemical fertilizer. At the same time, resource allocation, technological progress, and disaster areas had a negative impact on the use of chemical fertilizers. While deepening the division of labor and grain output had a positive role in promoting the total use of chemical fertilizers, with impact effects of 0.063 and 0.266, respectively. From the perspective of the mechanism, agricultural socialized services reduced the application of chemical fertilizer through resource allocation and scientific and technological progress. The deepening of the labor division had a 0.039 promotion effect on this path, and the government had a negative impact of 0.271 and an inhibition of 0.017 on the reduction of chemical fertilizers and this path, respectively. Further research found that there were differences between agricultural service scale, agricultural development degree, and geographical location in the impact of agricultural socialized services on reducing chemical fertilizer. Provinces with small service scales had a greater impact on the reduction of chemical fertilizers. Agricultural provinces have a higher impact on the reduction of chemical fertilizers than non-agricultural provinces on the premise of a more complete agricultural infrastructure and a more mature industrial chain. Socialized agricultural services in the central and western province had no significant impact on the use of chemical fertilizers. Among them, the lack of a supervision system and the lack of a deepening division of labor are the main problems faced by China’s socialized agricultural services, and they are also important factors that hinder the realization of fertilizer reduction. Therefore, we should continue to play the positive role of agricultural socialized services in reducing the amount of fertilizer, expanding the market demand for socialized services and green agricultural products, improving the deepening and supervision system of socialized service division, establishing a sound service outsourcing and green production standard system, strengthening risk management and control, promoting rational allocation of resources, and realizing an increase in both the income and productivity of farmers.