Abstract: The optical waveguide needs high quality optical substrate or high optical homogeneity.In our experimental research,the phase-matching temperature of MgO:LiNbO3 crystal has the relative to the refractive index and its temperature grade.According to the measuring results of phase-matching temperature and temperature grade of redractive index,the refractive index could be calculated.In the experimental system,three specimens taken from different position of a single MgO:LiNbO3 crystal set in a line and a 1.06μm laser source was used to measure the phase-matching temperature,finally the grade of the refractive index was calculated.