Ensuring water and food security under climate change is crucial for China's grain production. Using provincial data for 1995-2022, we simulated spatiotemporal dynamics of blue and green water footprints (WF), available water resources (AWR), and water footprint scarcity (WFS) in China's grain production. Nationally, total WF increased slightly (3.90*1011 m3 to 4.76*1011 m3), mainly due to expanded cultivated area and yields. The green water footprint (WFgreen), dominated by rice, was 1.6 to 2.4 times larger than the blue water footprint (WFblue) predominantly from maize. Spatially, the WF increased in northern China but decreased in the south, with major contributions from eastern and northern provinces such as Henan and Heilongjiang. However, a distinct spatial heterogeneity emerged: WFgreen prevailed in the humid east and south, while WFblue dominated in the arid northwest. Driven by WF and AWR, China's grain production has faced increasing and high water scarcity, with WFS consistently above 0.6. This was more urgent in the North China Plain and the northwest (especially Ningxia and Henan provinces), where blue water scarcity (WFSblue) became critical. Conversely, southwestern and southeastern provinces showed moderate or lower WFS, benefiting from higher AWR and lower WF. To mitigate these risks, we recommended integrated strategies focusing on water-saving technologies, optimized irrigation, enhanced rainfall utilization, crop structure adjustment, and interregional grain trade. Future research should adopt life-cycle-and nexus-based approaches while explicitly incorporating interregional virtual water flows to refine the water scarcity assessment and inform actionable policy.