Since the reform and opening-up, China’s agricultural science & technology policies have played a significant role in promoting agricultural modernization and technological innovation. With the continuous development of the agricultural science & technology sector, policy diffusion has become a key pathway for advancing agricultural technological progress and innovation. Against this backdrop, this study aims to explore the diffusion patterns and characteristics of national agricultural science & technology policies since the reform and opening-up, providing references for the formulation and implementation of agricultural science & technology policies in the new era. Using 3,207 agricultural science & technology policy texts collected from the “PKULAW” database between 1978 and 2023 as the data source, the BERTopic model was employed to identify policy diffusion themes. The study analyzes the characteristics of China's agricultural science & technology policy diffusion from multiple perspectives, including the evolution of policies, hierarchical thematic features, thematic trend analysis, and policy diffusion pathways. The research finds that national, ministerial, and provincial policies maintain a high degree of consistency in areas such as basic research, technological innovation, technology promotion, and talent cultivation. Furthermore, as the policy hierarchy descends, the content of policies becomes increasingly detailed. The diffusion of agricultural science & technology policies is influenced by their developmental stages, particularly since the 18th National Congress of the Communist Party of China, during which agricultural science & technology policies entered an innovative development phase. During this period, diffusion activities have become more active, and the efficiency of policy transmission has significantly improved. Overall, China’s agricultural science & technology policies primarily exhibit a top-down hierarchical diffusion, while also demonstrating parallel diffusion among departments and regions at the same level, as well as bottom-up absorption and promotion. This study proposes a series of policy recommendations, including strengthening policy promotion and pressure transmission at the national level, stimulating policy innovation capabilities at ministerial and local levels, and enhancing collaborative linkages among policies at different levels. These recommendations aim to provide insights for optimizing the agricultural science & technology policy system in the new era.