Yellow sapphires with unstable color are widely available in the market. How to effectively identify the characteristics of color unstable yellow sapphires is a hot topic in gemological research. In this paper, color-changing experiment, Ultraviolet-visible spectroscopy and 3D fluorescence spectroscopy were employed to study spectral characteristics of color-stable yellow sapphires. The color-changing experiment revealed that a part of yellow sapphires from Sri Lanka have unstable color, showing a photochromic phenomenon. Short-wave ultraviolet light dyed the original color, while sunlight faded the original color. The color of yellow sapphire after ultraviolet light is composed of stable part and unstable part. Color-unstable sapphire in the “colored state” and the “faded state” both showed blue-violet region absorption in the UV-Vis spectrum, which was likely related to the charge transfer of O^2--Fe³⁺, causing the stable yellow coloration of sapphire. The UV-Vis spectrum of sapphire with obvious color changes showed a significant absorption enhancement in the blue-violet region in the “colored state” compared to the “faded state”, which might attribute to ultraviolet irradiation enhancing the charge transfer between O^2--Fe³⁺. The UV-Vis spectroscopy result showed that samples had a weak Fe-related absorption peak, consistent with low Fe content. The low Fe concentration in the yellow sapphire is not enough to produce a stable yellow in color. 3D fluorescence spectra results revealed that the samples in “colored state” and “faded state” had the same fluorescence center (excitation wavelength=325~335 nm and emission wavelength=560~570 nm). The fluorescence intensity in the “colored state” is much higher than that in the “faded state”. Iron-containing yellow sapphire has a fluorescent effect, and the characteristic fluorescent center can be used as a potential identification characteristic to identify color-unstable yellow sapphire. This paper comprehensively reports the spectral characteristics and possible color causes of unstable yellow sapphires, providing potential identification characteristic to identify color-unstable yellow sapphire and a theoretical basis for the subsequent color modification process.