Abstract: Visible near-infrared colorimetric thermometry system instruments have been widely used in scientific research and industrial high-temperature monitoring. Since the gray value range of the instrument is limited, the existing research usually splices the temperature measurement bandwidth under different system parameters (exposure time, aperture) to extend the temperature measurement range. However, the fitted curves of grayscale values and temperature of the two channels of the system are exponential functions, and the growth rate of the two curves differs significantly, which will lead to the problems of narrowing the temperature measurement bandwidth and poor linearity of the system sensitivity. In addition, the existing evaluation and analysis methods for the performance of colorimetric thermometry system are not perfect. Given the above problems, a set of visible light near-infrared colorimetric thermometry system is developed, and a method of gray value attenuation is proposed to expand the temperature measurement bandwidth of the system. After the calibration experiment with the blackbody furnace is carried out, the channel sensitivity, temperature measurement bandwidth and system sensitivity of the improved and the original systems are compared and analyzed. It is found that this method extends the temperature measurement bandwidth of the instrument by more than two times, and effectively improves the linearity of the system sensitivity. Finally, the system's overall performance is compared with other instruments to verify the practicability of the instrument.