Spectral complex refractive index of fly ash particles is an important parameter in the processing of scattering properties calculation. On the basis of theory about inversion of complex refraction index using transimission method, a novel method was proposed, in which fly ash particles were dispersed in the water to compose turbid liquid, and spectral transmissivity of turbid liquid was measured using visible spectrophotometer. Particles size distribution was measured using laser particles sizing. So complex refraction index of fly ash particles was inverted simply. The result indicated that no signifcant absorption appeared in the visible wavelength range for fly ash particles, but transmittance decreased with wavelength increasing. The inversion results from the proposed method agreed with KBr sampling method. The proposed method is simpler, and has fewer constraints, meanwhile the inverion wavelength will be expanded if using infrared spectrophotometer.