In the present paper, the high-temperature phase change of pure tobermorite was investigated by TGA/DSC, X-ray diffraction and infrared spectroscopy (IR) respectively. The DSC results showed that four interlayer water molecules were lost when they were heated at 300 ℃. As the temperature increased to 724 ℃, Si—O—H bonds were cleaved and dehydroxylation occurred. The XRD results showed that many diffraction peaks of tobermorite disappeared and the crystal structure was broken and collapsed. Then tobermorite tends to be disordered and amorphous. When the calcination temperature increased to 861 ℃, the disordered structure recombined to wollastonite, and the crystal structure became ordered and stable. Finally, the structure completely transformed to 2M-wollastonite at 1 000 ℃. It should include the process of high-temperature phase change of tobermorite: tobermorite→dehydration tobermorite→dehydroxylation tobermorite→wollastonite.