An energy transfer technique for 4-(2-pyridylazo) resorcinol (PAR)-vanadium(Ⅴ)and acridine orange(AO)was studied,and the optimum conditions of energy transfer system were also experimented.It was found that in citrate-Na₂HPO₄ buffer solution at pH=5.5,energy transfers from AO to vanadium(Ⅴ)-PAR complexes.A new method based on energy transfer fluorescence quenching for the determination of trace vanadium(Ⅴ)with AO-PAR-V(Ⅴ)was established.The equation of linear regression is ΔF=165.4c+2.5,and the determination range of vanadium is 0.012-0.5 μg·mL^-1,with detection limit of 0.004 5 μg·mL^-1.The correlation coefficient is R=0.998 5,and relative standard deviation is 0.6%.The fluorescence reaction is completed within 15 minutes,and relative fluorescence intensity remains unchanged for 2.5 hours.The influence of foreign ions on the determination of V (0.5 μg· mL^-1) was examined,with a related error of 5%.The method has been used in the determination of trace vanadium in biological samples with the relative error of 6.98%,which meets the requirements of trace analysis.