In order to widen the application of microcrystalline magnesite, the thermal decomposition behavior of the microcrystalline magnesite in Sichuan and Tibet area was firstly studied by thermogravimetric analysis. Then the effects of the calcination temperature, calcination time and particle size on the preparation of active MgO from microcrystalline magnesite were studied by orthogonal experimental design. At last, high purity sintered magnesia was prepared by two-step calcination, meanwhile the effects of the light burning temperature, particle size of the light-burned MgO powder, molding pressure and the hydration rate of light-burned MgO powder on the properties of high purity sintered magnesia were studied. The results show that: (1) the calcination temperature has the greatest influence on the activity of light-burned MgO, followed by the holding time and the particle size; the optimal process of light burning for preparing active MgO is the microcrystalline magnesite with particle size of 1-0.5 mm heat-treating at 700 ℃ for 2 h; under these conditions, the microcrystalline magnesite decomposes completely; the average grain size of the obtained product is about 21.4 nm, and its activity of CAA is 20.16 s; (2) the effect of the light burning temperature on the density of high purity sintered magnesia is not obvious; the fineness of the light burning MgO powder and molding pressure help to increase the density of sintered magnesia; the lower the hydration rate of the light-burned MgO, the higher the density of the sintered magnesia; the high-purity sintered magnesite with w(MgO)≥98% and bulk density≥3.40 g/cm³ can be prepared by the two-step calcination; the grains are fine with size of 30-200 μm and the impurity at grain boundaries is little.