Meteorological flash droughts (MFDs) typically precede agricultural flash droughts, with their propagation risk heightened by intensified land-atmosphere coupling under global warming. Although China is highly vulnerable to such events, systematic analyses of the spatiotemporal patterns and drivers of MFDs remain limited. Here, we employ the operationally used Meteorological Drought Composite Index (MCI) to characterize MFDs across China's climatic zones and seasons from 1980 to 2024. Our analysis reveals three principal findings: (1) Spatially, humid regions experience more frequent but shorter-duration MFDs, whereas onset speed exhibits no clear humidity dependence. (2) Temporally, while China overall shows declining MFD frequency, prolonged duration, and significantly slowed onset (99% CI), the humid region (HR) displays contrasting trends: increasing frequency (significant in western HR in summer at 95% CI and in eastern HR in spring at 90% CI), extended duration (most pronounced in summer), and significantly accelerated onset in eastern HR (95% CI). Notably, the HR saw concurrent rises in both frequency and duration during 2020-2024, amplifying ecological risks. (3) Mechanistically, in drier regions, evapotranspiration is soil moisture-limited during MFD onset, and coupled soil moisture-evapotranspiration suppression prolongs droughts. In humid regions, evapotranspiration shifts from energy-limited at onset to soil moisture-limited during recovery, resulting in shorter events. The duration of precipitation deficits emerges as a dominant driver, contributing over 30% to both MFD duration and onset speed. These insights advance the mechanistic understanding of flash droughts and provide a foundation for improved monitoring, early warning, and adaptive management across China.