For organic cultural relics such as paintings and textiles, a very typical fading problem is often discovered during their conservation and restoration. It is necessary to conduct mechanism research using scientific methodology since this is an irreversible change for dye molecules. Caesalpinia sappan L., known as sappanwood or brazilwood, is famous, worldwide, as a natural red dye, but it is very poor in light fastness. The aim of this research was to investigate the fading using accelerated photodegradation and speculate the photofading by-products in the form of simulated UV ageing experiments. The solution kinetics of brazilin and protosappanin B, which have been found in relics, were studied in solvent DMF, and 20 sets of UV accelerated ageing standards were obtained. The products were analyzed using UV-Vis diffuse reflection spectrometry, ultra-high-performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (UPLC-QTOF MS) and proton nuclear magnetic resonance spectrometry (¹H NMR). These tools were applied together to analyze the color change trends and ageing process. 17 different photodegradation silk samples dyed with sappanwood were analyzed using UPLC-QTOF MS in the negative ion mode in combination with chemometrics. The results show that the fading produced a hypsochromic shift of max peak positions in their reflectance spectra with light ageing. The hydroxyl group of quaternary carbon in brazilin tended to dehydrate and made a series of changes while protosappanin B was easily converted into several compounds such as protosappanin C and urolithin C during the photoageing process. The simulated fading textiles dyed with sappanwood could be generally divided into two differential groups and differential metabolites including brazilin, protosappanin B and C could be analyzed through principal component analysis. These overall factors eventually caused the color change. These results are expected to be helpful to the research on natural dyes in textile heritages.