Abstract: The fluorescence spectra of cerium(Ⅲ) emitted at 358 nm exited at 254 nm in sodium chloride medium. A new FIA method using fluorescent quenching for the determination of phosphate was proposed. It was based on the ability of phosphate to inhibit the formation of a strongly fluorescent complex of cerium(Ⅲ) with NaCl in (CH₂)₆N₄-HCl buffer solution (pH=5.4), for the electrostatic interaction of the positive species of Ce(Ⅲ) in water with the negative charge of phosphate. With the analysis model Ln(F₀/F)=LnK_MF+LnK_e-LnK_F+LnM based fluorescent quenching, optimal chemical conditions for fluorescent quenching of the CeCl_n^(3-n)- (n=1～6) positive species by phosphates in samples were studied. The measurement throughput was 50 h^-1, and the concentration ranges of sodium chloride were 0.20～0.30 mol/L. Within the range of H₂PO₄^- from 0.20 to 1.80 μg/mL, calibration graph was obtained and the limit of detection was 0.003 μg/mL. The recoveries of artificial samples ranged from 97.3% to 99.0%.