The identification of supplementary cultivated land as a reserve resource is of great significance for ensuring implementation of the new mechanism of land occupation and compensation balance in China. Using Jiangsu Province as a case study, here, we use a “multi-period land use change patterns–multi-scenario land use simulation–cultivation suitability evaluation–identification of supplementary cultivated land” framework to explore identification of supplementary cultivated land. A single land use dynamic index and a land use transfer matrix were used to analyze land use pattern changes in Jiangsu Province and showed that the area of cultivated land in Jiangsu Province decreased significantly, mainly by being converted into land used for buildings, and waters and conservancy facilities. A Markov-FLUS model was used to simulate and predict land use quantity and spatial distribution under four scenarios: an inertial development scenario, a cultivated land protection scenario, an economic development priority scenario, and an ecological protection priority scenario. Sixteen factor indicators were selected from the four dimensions of natural land quality, social economy, management, and the ecological condition of the land, and the degree of suitability of cultivated land in Jiangsu was evaluated by multi-factor stepwise correction. The southern and central parts of Jiangsu had higher suitability, while the northern part had lower suitability. By superimposing these data on current land use data from 2023, the plots of land that were converted to or from cultivated land were identified. Combined with the suitability degree, the potential three major categories and eight types of sources for supplementary cultivated land, totaling 29,015.92 km², were identified, along with their distribution. A time sequence arrangement for these sources was initially set up. Corresponding management suggestions were proposed based on the adaptability of different supplementary cultivated land sources, with the aim of providing scientific references for the acquisition of supplementary cultivated land sources in the implementation of the national and local government’s farmland balance management.