Abstract: Agricultural machinery outsourcing operation service is an important part of China’s agricultural socialization services, which is important for improving agricultural production efficiency and promoting agricultural modernization. Based on the research data of 975 farmers in Sichuan, Jilin and Jiangsu provinces, this paper empirically tested the influence of social networks on the adoption behavior of farm machinery outsourcing services and its mechanism of action by using Heckman two-stage model and mediating effect model. The results showed that in the process of land preparation and harvesting, social networks could not only encourage farmers to adopt agricultural machinery outsourcing services, but also enhance their adoption level. Specifically, for every 1% increase in social networks, the adoption rates of agricultural machinery outsourcing services for farmers in land preparation and harvesting processes could increase by 53.1% and 57.9%, respectively, and the adoption level could increase by 6% and 7.7%, respectively. Different types of social networks had differentiated impacts on the adoption behavior of agricultural machinery outsourcing services by farmers. Vertical social networks had the highest level of impact, every 1% increase could increase the adoption rates of farmers in the land preparation and harvesting stages by 35.3% and 47.4%, respectively, and an increase of the adoption level by 4.5% and 7.5%, respectively. The impact of oblique social networks is secondary, with every 1% increase promoting an increase in adoption rates of 6.8% and 10.8% for farmers in the land preparation and harvesting stages, and an increase in adoption level of 1.1% and 1%, respectively. The impact of horizontal social networks was the weakest, with every 1% increase promoting an increase in adoption rates of 1.8% and 1.3% for farmers in the land preparation and harvesting stages, and an increase in adoption level of 0.1% and 0.1%, respectively. Risk preference plays a partly mediating role in the relationship between social networks and farmers’ adoption behavior of farm machinery outsourcing services. Accordingly, this paper proposes to cultivate and broaden farmers’ vertical and horizontal social networks, improve the platform of rural land management rights transfer, and enhance the development level of agricultural service outsourcing market.