The purpose of the article is to analyze the interaction effect between social network and extension service in farmers' agricultural technology adoption efficiency (TAE). The empirical analysis refers to samples of farmers' water-saving irrigation technology (WSIT) adoption from Minqin, China. The result indicates that social network and extension service can improve farmers' TAE, but they are found to be competitive from the perspective of overall social network. While from the perspective of four dimensions of social network, interaction and trust have positive effects on farmers' TAE, and learning and trust are found to be competitive with extension service. The policy implication describes as follows. First, government should pay attention to farmers' social learning and demonstration effect. And series of measures should be taken to cultivate farmers' social network. Second, extension service should be improved to satisfy farmers' multiple demand on agricultural technology. Third, government should combine the effects of social network and extension service, and make different promotion strategies for different regions. In addition, other influential factors cannot be ignored.