Synchronous Monitoring Atmospheric Corrector (SMAC), a support for the main camera, was one of the mayor facilities of high resolution and multi-mode imaging satellite. SMAC was designed to achieve atmospheric correction for high spatial resolution images based on radiative transfer model by getting synchronous parameters of aerosols and water vapor. SMAC had a special channel, where the radiance could be mostly absorbed by water vapor, for remote sensing of cirrus cloud. This channel was sensitive to the fluctuation of water vapor in the lab while stability measurement, thus the real performance of this channel could be covered up. The influence of water vapor fluctuation on stability test data was suppressed by controlling water vapor and setting synchronous monitoring detector. The results show that the stability error of SMAC’s channel for cirrus cloud detection decreased from 4.08% to 0.23%. It indicates that the stability measurement method of cirrus cloud diagnosing band for SMAC is reasonable and effective, which could reflect the stability performance of the product, and has important guidance significance for stability measurement of similar instruments.