The refractive index is the basic parameter of optical crystal, and the accurate determination of refractive index of crystal can provide basic calculation parameters and experimental parameters for electro-optic, acousto-optic and nonlinear applications of crystal. In order to accurately measure the refractive index coefficient of Pb(Mg1/3Nb2/3)O3-PbTiO3(PMN-PT) crystal in visible and near-infrared wavelengths, a laser autocollimatiion refractive index measurement system was set up.The error between the refractive index measurement results of pure lithium niobate crystal and the existing literature reports is less than 1/1 000. Using this system, the single-domain Pb(Mg1/3Nb2/3)O3-0.39PbTiO3(PMN-0.39PT) crystals with <001> polarization was measured. The refractive index and dispersion equations of single-domain PMN-0.39PT crystals at wavelengths of 594 nm,633 nm,1 150 nm and 1 520 nm were obtained,and the measurement results are consistent with the regularity of the existing literature. The results show that this method can be used to determine the refractive index of new type electro-optic crystal accurately and quickly.