Abstract: In the context of the rapid decline in biodiversity, the biodiversity of nature reserves has received more attention, whereas attention to agricultural biodiversity is relatively scarce, and there is a lack of a rapid method for evaluating farmland biodiversity. This study used Jiyang District, Sanya City, Hainan Province as a case study and developed a method for rapidly evaluating the biodiversity of county-level farmland. This method is based on a landscape distribution map of county-level farmland. Twelve representative rice plots were selected to ensure independence and repeatability. During the peak growth period of crops, vascular plants, arthropods in the vegetation layer, and birds were investigated as representative groups of farmland biodiversity. The Shannon index for each group in each sample plot was calculated based on survey results. Subsequently, through normalization calculations, the Shannon index integrating the three biological groups was obtained. Based on these results, an approximate analysis of the farmland biodiversity of each sample plot was conducted. Finally, the data were sorted, and the Shannon index of the entire county-level farmland habitat in Jiyang District was calculated through normalization. Relevant data for each group in the natural habitats of Jiyang District and its surrounding areas, as well as relevant literature data for each group in the farmland habitats of Jiyang District, were searched. Differences in biodiversity between natural and farmland habitats were compared using equations and classification standards designed by the author. Using this method, the level of biodiversity of the farmland habitats in Jiyang District was evaluated, which reflects the diversity of the farmland ecosystem, the degree of human interference, the potential for ecological restoration, and other situations. The results showed that the plant community structure of paddy fields in Jiyang District was relatively simple. Furthermore, there were many invasive plants, and agricultural ecological land was scarce. Arthropod biodiversity was relatively rich; however, the beneficial-to-harmful ratio in most sample plots was low. The structure of the bird community was relatively simple, and the species diversity was low. Overall, farmland biodiversity in Jiyang District was low, which may have been caused by the proliferation of invasive organisms and long-term intensive management. Compared with nature reserves, the biodiversity resources of farmland habitats in Jiyang District was less than 10% of those in nature reserves, especially in terms of plant diversity, while the biodiversity resources of arthropods was relatively high. The method proposed in this study can comprehensively and rapidly evaluate the biodiversity of county-level farmlands using a single survey.