Abstract: A rare earth ternary europium ion complex with fluorescent lifetime of 0.544 0 ms was synthesized by using 4,7-diphenyl-1,10-phenanthroline and 2-thenoyltrifluoroacetone as synergic agents and β-diketone ligands. The complex was used as fluorescent probe for pH determination of environmental water samples by time-resolved fluorescence analysis. The fluorescent probe showed the maximum and stable time-resolved fluorescence intensity when the pH ranged from 7.00 to 8.95, while the time-resolved fluorescence intensity changed obviously in acidic (pH<7.00) and strong alkaline (pH>8.95) conditions. The time-resolved fluorescence quenching of the fluorescent probe showed linear response toward pH in the range of 2.50-7.00 or 9.91-14.00. The prepared fluorescent probe was insensitive to dissolved oxygen and ionic strength. The prepared fluorescent probe was used to determine pH of real water samples, giving results in consistent with those obtained by glass electrode and RSDs (n=8) in the range of 4.3%-8.1%.