The thermal refractive index coefficients were obtained at these wavelengths and the constants of modified Sellmeier's equation were also obtained at the temperature range.

According to the measured values of the refractive indices of the Nb∶KTP crystal at different temperature, the expression of the thermal refractive index coefficients as a function of the wavelength is got.

For some crystals,one or more of the thermal expansion coefficients and thermal refractive index coefficients have an opposite sign to the other coefficients such that a direction of temperature insensitive optical path length exists.

The Sellmeier's equation and the thermal refractive index coefficients for this kind of crystal were obtained.

On the basis of measured values and modified Sellmeier s equatins, the expressions of thermal refractive index coefficients of this kind of crystal are derived in this PSPer.

The automatic high-precision system, which is composed of temperature control system,precision goniometer system, optical system and electrical control system for measuring the temperature coefficient of refractive index of infrared materials is introduced in the paper.

The refractive index coefficient of the material obtained was -1.