In this article, a sapphire fiber colorimetric thermometry system is used to achieve high-precision measurement of ultrahigh temperature up to 2000 K by correcting the reflected radiation error. Based on the principle of colorimetric temperature measurement, a radiometric thermometry system is constructed using sapphire fiber with a high melting point. Reflection errors on ceramic blades in high-temperature furnaces are eliminated by calibrating the reflected radiation. The experimental results show that the temperature measurement error of the sapphire fiber colorimetric thermometry system is less than 1% in the range of 1400–2000 K. And the repeatability error of the experiment is ±0.9%, which achieves the high-precision measurement of ultrahigh temperature by correcting the reflection error.