A novel coumarinylhydrazone fluorescent probe L was designed and synthesized from 4-(diethylamino)salicylaldehyde, its structure was characterized by NMR, IR. Fluorescence emission spectra showed that in ethanol solution, probe L could form a 1:1 complex L-Cu 2+ with Cu 2+ to realize the "turn-off" detection of Cu 2+ with high specificity and sensitivity (3.7 × 10 -7 mol/L). Meanwhile, the complex L-Cu 2+ had a specific fluorescence-enhanced response to HPO 4 2- with a detection limits down to 5.6 × 10 -7 mol/L and was resistant to the effects of many common anions (NO 2 - , N 3 - , CO 3 2- , SO 3 2- , HPO 4 2- , I - , Br - , F - , HCO 3 - , SO 4 2- , NO 3 - , Cl - , CH 3 COO - , Cr 2 O 7 2- , S 2 O 3 2- , P 2 O 7 4- ). Detection mechanism could be HPO 4 2- captured Cu 2+ of the complex and released the free ligand L. At last, the complex L-Cu 2+ was successfully applied to the determination of HPO 4 2- in different environmental water samples, and the spiked recoveries ranged from 98.05% to 108.18% and the relative standard deviations of 0.75% ~ 2.9%, which had good application prospects.