Abstract: As a rare gemstone, cassiterite was once called "Yunnan diamond" because of its strong fire. There are many researches on the geological characteristics of cassiterite and its flotation in China, but the thermochromism phenomenon of cassiterite has not yet been reported. In this study, conventional gemmological instruments, Raman spectrometer, laser ablation plasma mass spectrometer (LA-ICP-MS) and UV-Vis spectrometer were used to investigate the mechanism of the thermochromic phenomenon of cassiterite from Yunnan Province, China. The results showed that the main component of cassiterite is tin dioxide, with an average content of up to 99.4 wt%, and it contains trace elements Fe, Ti, Si, Ni, Ta, Nb, et al. In the process of increasing from room temperature to 500 ℃, the crystal lattice of cassiterite expanded, resulting in the reduction of crystal bond force constant and band gap value, which leaded to a shift of the in situ high-temperature Raman peaks of cassiterite toward lower frequencies, and caused the absorption edge of the UV-Vis absorption spectrum of cassiterite to redshift from 380 nm to 450 nm, cassiterite changed from nearly colorless to yellow. In the process of cooling from 500 ℃ to room temperature, the crystal lattice shrinked, which increased the crystal bond force constant and band gap value. The in situ high-temperature Raman peaks of cassiterite regressed toward high frequencies, the absorption edge of UV-Vis absorption spectrum continuously blueshifted back to 380 nm, and the sample colour returned to nearly colorless. The optical band gap values of tin dioxide crystal during heating and cooling processes have been calculated. It is found that the bandgap value E_g of cassiterite from Yunnan decreased from 3.28 eV to 2.75 eV with the increase of temperature. In conclusion, the change of crystal band gap is due to lattice expansion and contraction caused by temperature change is the reason for the thermochromism of cassiterite from Yunnan.