Abstract: In order to study nonlinear refractive index of bismuth telluride, a 3-D topological insulator, nonlinear refractive index of 800nm femtosecond pulse laser was measured by reflection z-scan method. After theoretical calculation and experimental data fitting, nonlinear refractive index of bismuth telluride crystal could reach the order of magnitude of 10-14m2/W, 105 times of that of quartz. Nonlinear refractive index decreased with the increase of incident power. Until peak intensity reached 85GW/cm2, nonlinear refractive index tended to be a constant. The results show that bismuth telluride is a remarkable nonlinear optical material which has huge potential in the application of all-optical signal processing and optical switching.