The changes of phase and structure of low-grade clay at different calcination temperatures (650, 700, 750, 800, 850 ℃) were studied by DSC-TG, XRD, IR, Raman and NMR. The results show that the low-grade clay is mainly composed of quartz with high crystallinity, and contains a small amount of phengite, kaolinite, illite, calcite, pyrite and microcline with low crystallinity. After calcination at 650~850 ℃, kaolinite and illite decompose into amorphization SiO2 and Al2O3, and some quartz transform into amorphous SiO2. The adsorbed water and the structure water in the low-grade clay decompose, the aluminoxy octahedron gradually transforms into aluminoxy tetrahedron, and the short-range order structure of Si-O-Si in quartz is also destroyed to a certain extent.