Theory of two-color Z-scan was studied in this paper.The expression of the normalized on-axis transmittance in the far field was derived with some simplifications,and the influences of the wavelength and beam waist radius of pump and probe beams on the measurement sensitivity were discussed. Utilizing Ar+laser with the wavelength of 514.5 nm as Pump beam and He-Ne laser with the wavelength of 632. 8 nm as probe beam,we performed two-color Z-scan experiment in organic solvents of C6o and 4-BCMU which had different absorption coefficients at 514.5 nm, and their thermal-induced nondegenerate nonlinear refractive index n2 were measured to be -3.46×10-5 esu and -4.06×10-5 esu, respectively. Some phenomena observed in this experiment were explained.