The principal refractive indices of crystal CsB3O5 (CBO) at the wavelengths of 0.266, 0.355, 0.532, 0.473, 0.633, 1.064, and 1.338 μm are accurately measured by using the autocollimation method within the temperature range from 40°C to 190°C. We derive and report equations of thermal refractive index coefficients as a function of wavelength that could be used to calculate the principal thermal refractive indices at different wavelengths. Sellmeier equations at different temperatures were derived and used to calculate the phase-matching angles for the third-harmonic generation and the fourth-harmonic generation of CBO crystal at different temperatures.