Calcium is a key regulator of mitochondrial function under both normal and pathological conditions. The mechanisms linking metabolic activity to mitochondrial Ca signaling remain elusive， however. Here， by monitoring mitochondrial Ca transients while manipulating mitochondrial membrane potential (ΔΨ)， we found that mild fluctuations in ΔΨ， which do not affect Ca influx， are sufficient to strongly regulate NCLX， the major efflux pathway of Ca from the mitochondria. Phosphorylation of NCLX or expression of phosphomimicking mutant (S258D) rescued NCLX activity from ΔΨ-driven allosteric inhibition. By screening ΔΨ sensitivity of NCLX mutants， we also identified amino acid residues that， through functional interaction with Ser258， control NCLX regulation. Finally， we find that glucose-driven ΔΨ changes in pancreatic β-cells control mitochondrial Ca signaling primarily via NCLX regulation. Our results identify a feedback control between metabolic activity and mitochondrial Ca signaling and the "safety valve" NCLX phosphorylation that can rescue Ca efflux in depolarized mitochondria.