The integration of the three agricultural industries and the development of modern agriculture are of great significance for bridging the development gap between rural and urban areas, and for achieving fully coordinated development of both urban and rural areas. They are also the important contents of building the modern agricultural industry system, of realizing the rural revitalization, and of accelerating the integrated development of urban and rural areas. Based on the analysis of the necessity and the feasibility of the integration of the three agricultural industries and modern agricultural development, and from the perspectives of market resource allocation, social entity participation and government system guarantee, this paper analyzed and discussed the mechanism of the integration of the three agricultural industries and the development of modern agriculture. Results show that the integration of the three agricultural industries and the development of modern agriculture play important roles in accelerating the reform of agricultural supply side, revitalizing the rural economy, and promoting farmers’ income. The integration of the three agricultural industries has accelerated the development of modern agriculture; while the development of modern agriculture provides the basis for the integration of the three agricultural industries. The two interact and interrelate with each other, and are inseparable. Therefore, to further increase agricultural product quality and production efficiency and to promote the integration of the three agricultural industries and the development of modern agriculture, this paper provides the following suggestions: to innovate the development model of agriculture, to strengthen the cultivation of new agribusiness entities, to perfect the free and equal flow of input resources between urban and rural industries, and to enhance policy supports from the local governments.