In order to reduce the shrinkage cracking of mass concrete, MgO expansive agent was made by calcinating magnesite tailings. The effects of different calcination temperatures on the mineral composition, particle size and activity of MgO expansive agent were studied. The changes of mineral composition, hydration process, thermal stability and micro morphology of cement mortar with MgO expansive agents were studied by means of X ray diffraction, scanning electron microscope and thermal analyzer. The results show that under the condition of complete magnesite decomposition(calcination temperature over 1000℃, and constant temperature for 1h), with the increase of calcination temperature the grain size of magnesium oxide increases from 276nm to 1380nm, the activity decreases from 708% to 33%, and the hydration rate slows down. There is a linear relationship between the grain size of magnesium oxide and the reactivity. MgO activity affects the expansion limit of cement mortar. The expansion limit of cement mortar mixed with MgO expansive agent made of calcination temperature 1000℃ is the largest. Under the condition of 20℃ water curing, the expansion limit of 7, 28,120d can reach 0022%, 0037% and 0062%, respectively. In addition, the hydrated product magnesium hydroxide in the system shows lamellar growth in the matrix.