Abstract: Hydration rate is a key index for evaluating the active level of MgO in preparation of active MgO with calcined Magnesite. But it is only suitable for the calcined product with impurity content of less than 2%. With depletion increasing of high-quality magnesite resources，and in order to adjust to the exploration of the high impurity of magnesite，the concept of the corrected hydration rate is proposed. Taking a magnesite ore from Liaoning Haicheng as raw material，the technical conditioning tests for the preparation of active MgO by calcination were studied. The results showed the corrected hydration rate can more objectively and accurately reflect the activity of MgO in calcine process than the hydration rate. This conclusion is coincide with the XRD，SEM analysis of the calcined product and its hydrates.

Key words: Magnesite, Calcined, Active MgO, Hydration rate, Corrected hydration rate