The oil pollutants detector is designed in this paper. The pulse xenon lamp is used as light source; the step type multi-mode pure silica fiber is chosen to transmit the excitation and emission light. The asymmetric Czemy-Turner light path of high precision grating monochromator is adopted. The detector is applied to determine the fluorescence spectrum of diesel, gasoline and kerosene. The optimal excitation /emission wavelengths are: 290/330 nm (diesel), 270/300 nm (gasoline) and 280/330 nm (kerosene). The detection limits are: diesel (0.025 mg·L-1), gasoline (0.042 mg·L-1) and kerosene(0.054 mg·L-1). The relative errors are: diesel(2.55%), gasoline(2.06%) and kerosene(1.71%). Experiment results show that the designed detector has high accuracy of measurement. The different concentration of diesel, gasoline and kerosene mixed solution is configured, and three dimensional fluorescence spectra being measured. The self-weighted alternating trilinear decomposition is adopted to decompose the spectrum data. The predicted concentration and recovery rate show that self-weighted alternating trilinear decomposition has high resolution for mixed oil substance.