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Land transformation during global urbanization has led to a sharp decrease in farmland, causing not only food security issues but also ecological problems. To address this issue, the Chinese government has implemented the Requisition–Compensation Balance Policy for Farmland (RCBF) in 1997. This policy effectively curbed the reduction of farmland, but the compensated land often comes from mountainous and desert areas, leading to fragmentation of farmland and subsequent ecological security issues. The balance between farmland requisition and compensation is closely related to ecological security. However, current research on farmland occupation and compensation is mostly based on farmland area. The area occupied and compensated for by farmland in different regions is inconsistent, and using only net increase or decrease in area to represent farmland occupation and compensation cannot accurately and fairly compare the degree of farmland occupation and compensation between regions. Therefore, this study has proposed a novel index to measure the balance of farmland requisition and compensation—the Farmland Requisition and Compensation Index (FOCI). FOCI can transform dimensional expressions that represent the area of farmland occupation and compensation into dimensionless expressions, namely scalars, which makes it possible to visually and fairly compare the degree of farmland occupation in each region. Then, this new index has been used to investigate the spatiotemporal evolution of farmland requisition and compensation in China at the national and provincial levels over the past 30 years (1990–2021), as well as the impact of this change on the fragmentation of farmland landscape and ecological service value. The results indicate that (1) FOCI shows a trend of first decreasing and then increasing at both national and provincial scales; (2) Provinces with increasing FOCI are mainly concentrated in the southeast and northwest regions, while significant decreases in FOCI are observed in the southwest region, indicating a shift of the FOCI center of gravity towards the southeast; (3) FOCI and farmland landscape fragmentation are significantly positively correlated spatially, suggesting that provinces with higher levels of farmland requisition and compensation also exhibit higher levels of farmland landscape fragmentation; (4) FOCI and ecological service value are significantly negatively correlated spatially, indicating that provinces with higher levels of farmland requisition and compensation have lower ecological service values, with these areas mainly concentrated in the northwest region of China. In general, FOCI has the advantage of eliminating the dimensional influence in different regions and could be a reliable alternative for evaluating the balance of farmland requisition and compensation between regions.