Based on the Kirchhoff law, a practical dual-wavelength high-precision fiber-optic colorimeter using LiTaO3 pyroelectric detector is presented. According to the relationships between various technical parameters and main specifications (temperature resolving power of single detector, linearity of R(T)～T curves, temperature measurement sensitivity and relative temperature sensitivity), a method of optimal parameter selection for determining two operating wavelengths of a new kind of the practical IR fiber-optic colorimeter using LiTaO3 pyroelectric detector is presented. Experimental results show that temperature measurement sensitivity exceeds 1.0×10-4 ℃-1, and relative tomperature sensitivity and temperature measurement accuracy are better than 0.8 and 0.15% respectively at 400～1300 ℃ and λ1=2.1 μm, λ2=2.3 μm which reach design requirements.