Abstract: Farmland biodiversity is a core component of global biodiversity, which is essential for maintaining the stability of farmland ecosystems and ensuring the security of agricultural production. However, current research has paid insufficient attention to agricultural biodiversity, and there is a lack of a rapid evaluation method for farmland biodiversity. Taking Jiyang District of Sanya City, Hainan Province as the case study area, this study developed a rapid evaluation method for county-level farmland biodiversity, to provide a reference for the investigation and assessment of farmland biodiversity at the county scale. Based on the county-level farmland landscape distribution map, this method selected 12 representative rice field plots, and carried out field surveys during the peak growth period of crops using vascular plants, arthropods in the vegetation layer, and birds as representative groups. The Shannon diversity index of each group in each plot was calculated, and the comprehensive Shannon diversity index was obtained through normalization. An evaluation formula for the relative difference in biodiversity between farmland and natural habitats R=(z/Z+j/J+n/N)×1/3 was innovatively constructed, along with a 6-level grading standard, to complete the comprehensive evaluation of county-level farmland biodiversity. The results showed that: a total of 29 species of vascular plants were recorded in the farmland habitat of Jiyang District, among which invasive plants accounted for 34.38%, with a simple community structure; 3 579 arthropod individuals were collected, belonging to 9 orders and 67 families, with relatively rich family-level diversity but a low ratio of beneficial to harmful organisms in most plots; 19 bird species were recorded, with a simple community structure and high concentration of dominant species; the relative value R of comprehensive biodiversity in farmland habitats was 9.81%, corresponding to the “medium” level, and its biodiversity resources were less than 10% of those in the surrounding nature reserves. This method can realize a comprehensive and rapid evaluation of county-level farmland biodiversity through a single survey with simple operation, and can provide a scientific basis for the conservation of agricultural biodiversity and the refined management of farmland ecosystems.