In view of synchronous fluorescence possessing the character of good selectivity, high sensitivity, less interference, etc. it can be used for simultaneous determination of multi-component mixtures of polycyclic aromatic hydrocarbons(PAHs). A new method of constant-wavelength synchronous fluorescence spectrometry to determine two naphthalene and phenanthrene of PAHs was developed in this study. The effect of different experimental conditions, such as different disolvents for character of fluorescence spectra and the choose of the optimal wavelength difference were studied. Experiment showed that the simultaneous indentification and quantitative determination of the two PAHs when Δλ=100 was chosen. The fluorescence intensity was linearly related to naphthalene and phenanthrene concentration in the range of 0.5～25.0 μg·L-1 with correlation coefficient 0.999 5 and 0.999 7, respectively. The detection limits were all lower than 0.03 μg·L-1, and the relative standard deviations for naphthalene and phenanthrene were 1.19% and 180%(n=7), respectively. Results show that the compounds can be analyzed qualitatively and quantitatively by synchronous fluorescence spectrometry.