In order to study the degradation of indigo carmine, we establish the indigo carmine photo degration rate model by the UV-vis absorption spectroscopy. The absorbance at 611 nm of the absorption spectra of indigo carmine solution declines with the increase of illumination time and up to 0 after 8 hours. Based on the absorption spectra of the indigo carmine solutions with different mass concentrations, we established an absorbance-mass concentration prediction model at 611 nm and a peak area-mass concentration prediction model within 500-700 nm by exponential fitting. The results show that the peak area-mass concentration prediction model is more accurate than the former. Taking the completely degraded indigo carmine as interferent, we mixed it with the newly-prepared indigo carmine solutions to verify the peak area-mass concentration prediction model, and the model''s average relative deviation is 2.7%. Combining the absorption spectra area of the indigo carmine solutions (within 500-700 nm) at different illumination durations with the peak area-mass concentration prediction model, we obtain the content of indigo carmine and degradation rate, and obtain an exponential function between the degradation ratio of indigo carmine and the illumination time. The degradation rate model established in this paper has high measurement accuracy, and it can easily test the degradation rate of indigo carmine.