Based on the Kirchhoff law, a practical high-precision dual-wavelength fiber-optic temperature measuring system using LiTaO3 pyroelectric detector is presented. The structure and the principle of the system are described. According to the temperature resolving power of single detector, the linearity of R(T)-T curves, temperature measuring sensitivity in the measuring system as well as the relationship between the temperature measuring errors and the change of wavelength bandwidth, and after considering the minimal responsibility and the measuring resolution of a single detector, the optimization for its working wavelength bandwidth was done. The optimal result showed that the proper bandwidth of the working wavelength should be 20nm at 400～1300℃ and λ1=2.1 μm, λ2=2.3 μm.