Abstract: On the basis of theory of light and lots of experiments, the proper light wavelength is determined. In the wave band, oil samples imbibe light with general absorp tion, intensity of penetrating light is more stable and higher relatively, variations of oil viscosity have little effect upon ratio of penetrating light,and change of oil color influ-ences ratio of penetrating light.With the Lambert-Beer law combined with the system of single source and double laser-lights, theoretic foundation is inferred for quantitative analy-sis of oil contamination, eliminating influence of oil color and improving accuracy of the device.