| Summary: | The purpose of this study is to develop a new remote sensing method to assess the area of cultivated land illegally converted to nurseries and orchards in the Yangtze River Delta region of China over the past 40 years (1980–2020), and then estimate the increase in grain yield based on this area. Our result showed that 2.23 × 10<sup>4</sup> km<sup>2</sup> of cultivated land was reduced by 11.8% over the past 40 years. About 14,521.40 km<sup>2</sup> of cultivated land was converted to nursery and orchard from 1980 to 2020. The conversion area was unevenly distributed among different administrative regions. Zhejiang and Anhui Provinces had higher conversion rate and area than Jiangsu Province and Shanghai Municipalities. After the illegal nurseries and orchards return to cultivated land, if rotation farming mode is adopted, in which each grain crop is planted only once a year, the increased production of rice, wheat, corn, beans and tubers will reach (632.57 ± 13.08) × 10<sup>4</sup>, (0~531.05 ± 33.25) × 10<sup>4</sup>, (0~556.19 ± 30.36) × 10<sup>4</sup>, (0~249.85 ± 13.14) × 10<sup>4</sup> and (0~489.11 ± 28.14) × 10<sup>4</sup> tons at most in each year, respectively. Our results provide theoretical guidance for implementing food security policies and alleviating contradiction between grain production and rural labor shortages in the Yangtze River Delta region.
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