The development of sustainable agriculture is an important link in promoting agricultural modernization. As a new sustainable agricultural production model, the promotion effect of agricultural production trust depends on the participation of farmers. Therefore, based on the micro-data of 461 valid questionnaires from farmers in the Heilongjiang Province and the Heckman sample selection model, this study empirically analyzed the impact of social capital on the decision behavior of farmers to adopt custody services from two aspects of adoption behavior and adoption degree. The results showed that social capital had a significant positive impact on the adoption behavior and the degree of adoption of rural households’ trusteeship services. From the perspective of different business scales, for large-scale households, participation in the service promotion activities of agricultural means of production suppliers and trust in the technical information disseminated by the government had a significant positive impact on the adoption of custody services. However, for small farmers, participating in village collective service promotion activities and trusting the service information spread by their neighbors’ relatives and friends had a significant positive impact on trust adoption behavior. Therefore, more attention should be paid to the impact of social capital on farmers’ trust adoption behavior decision making in the future agricultural trust service promotion process to accelerate the promotion of sustainable agricultural development.