Abstract: Cropland is highly vulnerable to climate variations and hydrothermal resources. Crop planting boundaries can be altered to reshape the production patterns in cropland. It is often required for the regional potential to the stable and efficient production. This study aims to explore the impacts of climate change on cropland use and their practical responses in cropping systems. Thereby, the cropland distribution was then optimized for the national food security. A systematic analysis was also made to determine the impacts of climate change on cropland use in China from 1980 to 2020. Hydrothermal conditions were evaluated in cropland, according to the data from 728 meteorological stations. The research period was divided into 1980–1990 and 2010–2020. Some changes were examined in the potential cropping systems and the northern planting boundaries of the major grain crops (wheat, maize, and rice). The response of the cropland use was analyzed to fit the prediction of three models—XGBoost, Random Forest, and Decision Tree—according to the remote sensing interpretation data with statistical yearbook data. The results included: (1) the accumulated temperature ≥0°C (AT0) increased at 6.05–16.13 °C·d/a, and ≥10°C (AT10) at 7.31–18.37 °C·d/a, with the prolonged thermal durations. Precipitation showed a 3.1% decreasing trend in the Huang-Huai-Hai Plain, while other regions exhibited increasing trends. (2) Potential single-cropping zones were expanded by 474,000 km², primarily in the Qinghai-Tibet Plateau and the rest areas that were previously constrained by low accumulated temperature. Potential double-cropping zones were expanded by ~93,000 km², and the northern boundary of triple-cropping zones was shifted northward by 192.5 km, thus covering an additional 474,000 km². The suitable planting area for the wheat was expanded northwestward by 38.4 km (47,400 km²), and that for rice/maize shifted northward by 19.4 km (53,800 km²). (3) The total cropland area increased gradually from 1.754 million km² in 1980 to 1.793 million km² in 2000 before declining to 1.767 million km² in 2020. The national MCI increased by 0.07, among which the Northeast Plain recorded the largest average growth rate of 0.63. But the outstanding declines were found in the Middle and Lower Yangtze Plain, Beijing-Tianjin-Hebei region, Sichuan Basin, and Shaanxi Province. In terms of the crop planting, the nationwide wheat sown area decreased by 18.9% with the sharp drops in Northeast China and the Sichuan Basin. But the slight growth was in the Huang-Huai-Hai Plain, while the rice planting area first declined, then stabilized with a surge in Northeast China and a decline in South China. Maize sown area increased by 105.4%, indicating a trend of "northward expansion and southern extension". (4) In regions with northward-shifted planting boundaries, 67 km² (0.07%) of cropland in Northeast China and 2817 km² (0.74%) on the Qinghai-Tibet Plateau was cultivated. In potential double-cropping expansion zones, the MCI increased from 1.03 to 1.08 in the Huang-Huai-Hai Plain, with nearly one-third of counties with the growth; It surged from 0.41 to 0.90 in Liaoning, while decreasing in the Loess Plateau, due to Qinling ecological protection. In the potential triple-cropping zones, the Sichuan Basin performed robustly, whereas the Middle and Lower Yangtze Plain showed a 0.22 decline in the MCI. (5) Thermophilic crops were dominant in Northeast China, with 280 maize-dominated counties. New rice-dominated areas emerged in Heilongjiang, and the wheat-growing regions were expanded eastward into Inner Mongolia. This finding can provide updated temporal insights to integrate the theoretical impacts with empirical changes. The critical and practical implications can also help assess the cropland use responses to climate change of food security and regional agriculture.