Ecosystem services (ES) link ecosystems and human societies, yet approaches to ecosystem service flow (ESF) are constrained by ambiguous definitions and limited process-oriented and scale-sensitive classifications. Here, we developed a "supply-demand-flow" framework distinguishing potential and actual supply and demand, and classifying ESF into in situ, interior, and exterior flows. Integrating multi-source datasets, biophysical models (InVEST, RUSLE, i-Tree), and socio-economic accounting based on population-consumption dynamics and sectoral statistics, we mapped nine ES across China from 2000 to 2020 at 1 km resolution. Results revealed pronounced ES supply and demand spatio-temporal heterogeneity. While the national ESF pattern remained stable, most services showed increasing exterior reliance, whereas water yield and tourism recreation became more locally sustained. Using flow balance and demand fulfillment as policy-proximal indicators, five management zones were delineated: local-sustained counties dominated southern China (similar to 30%); local-pressured types occurred in the Northeast, Qinghai-Tibet, and southwestern areas (similar to 15%); external-pressured types occurred in arid northwestern regions (1%-3%); southeastern coasts shifted toward external-sustained types (similar to 30%); and dynamic-transitional counties (similar to 20%) were scattered nationwide. By decomposing supply-demand relations into supply realization, spatial reallocation, and demand fulfillment, our framework helps align ecological functions with management priorities and offers insights for reconciling development and conservation in comparable socio-ecological contexts.