Optical thermometry is a research hotspot in current industrial combustion fields for flame monitoring and combustion diagnostics. A new temperature measurement method of optical double-color imaging based on grayscale inversion has been proposed. The functional block diagram of temperature measurement system has been given and the temperature measurement algorithm based on grayscale inversion has also been deduced. The analysis has demonstrated that, compared with the three-color thermometry, this thermometry has eliminated the negative effects of flame′s gray body hypothesis and non-ideal spectral responsive bandwidth, and overcome the drawbacks of nonlinear three-channel output and few energy share occupied by solar visible radiation. The research has shown that, this method has high temperature measurement accuracy, and can measure the 2D-temperature distribution of boiler flame. It can provide the good instruction for flame detection and combustion diagnosis, which will be widely used in future.