Activity is a main application characteristic that is closely related to the microstructure of light-burned magnesia. The light-burned magnesia was obtained via calcining macrocrystalline magnesite powder at 700-1 100 ℃. The cell parameter, grain size, crystallinity, specific surface area, pore diameter distribution, specific pore volume, average pore diameter, activity and microstructure of light-burned magnesia at different temperatures were characterized. The particulate structure of light-burned magnesia was investigated by thermogravimetry based on the non-isothermal reaction kinetics theory, and the mechanism of the particulate structure evolution with temperature and its influence on the activity of light-burned magnesia were further analyzed. The results show that the cell parameter a0 of light-burned magnesia firstly reduces and then becomes stable, and the grain size increases while the specific surface area decreases gradually when the calcination temperature increases from 700 ℃ to 1 100 ℃. However, the activity of light-burned magnesia does not comply with this rule, and reaches the maximum value at 800 ℃. The thermal decomposition of macrocrystalline magnesite powder is controlled via the chemical reaction at the phase interface. The decomposition product, i.e., the light-burned magnesia ‘pseudo’ particles, is composed of magnesia microcrystals and connected a pore network structure. The sintering of magnesia microcrystals affect the "pseudo" particle structure of light-burned magnesia as the calcination temperature increases, thus affectingits activity. The sample light-burned at 800 ℃ has a smaller grain size, a lower crystallinity, a larger specific pore volume, and a largest average pore size due to the lattice adjustment, which is conducive to the penetration of the reactant solution into the particles and accelerates the reaction progression, leading to the maximum activity.