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Abstract: ABSTRACT Astrocytes are recognized as key components of neurovascular coupling (NVC), yet whether and how astrocytic engagement in NVC depends on sensory stimulus duration remains unclear. Here, we investigated a duration‐dependent astrocytic mechanism underlying cerebral blood volume (CBV) modulation by comparing neurovascular responses to brief (5 s) and prolonged (20 s) sensory stimulation, using in vivo intrinsic optical signal (IOS) imaging and two‐photon microscopy combined with electrophysiology and vascular cannulation. We show that prolonged stimulation selectively recruits an integrated astrocytic signaling cascade involving aquaporin‐4 (AQP4)‐mediated volume changes, Ca 2+ influx through mechanosensitive transient receptor potential ankyrin 1 (TRPA1), and D‐serine release through the Ca 2+ ‐dependent Bestrophin‐1 (BEST1) channel. This signaling cascade drives sustained neuronal activity, leading to CBV modulation and sensory perception. Genetic ablation of each component in this pathway selectively impairs CBV responses, neuronal activity, and sensory behavior during prolonged stimulation, while exogenous D‐serine fully restores these deficits. We further demonstrate that tonic astrocytic D‐serine release via BEST1 directly regulates the basal tone of penetrating arterioles. These findings together identify astrocytic D‐serine as one of the key gliotransmitters that modulate CBV and neuronal activity during prolonged sensory demand and fine‐tune vascular tone in resting states.