Three methods were used to evaluate hydration reactivity of calcined magnesite products. Both conventional citric acid coloration reactivity test (index of CT) and MgO hydration conversion method (index of X) cannot properly evaluate the reactivity when the magnesite ore decomposes incompletely at lower calcining temperatures. Therefore, a corrected MgO hydration conversion method containing the index of X-corrected is proposed by considering the decomposition ratio of magnesite ore. Results show that decomposition ratio of magnesite ore is quite low at temperatures lower than 650 degrees C, which influences the reliability of the conventional reactivity test results (index of CT or X). Reactivity of MgO decreased as the calcining temperature and time increased when the index of X-corrected was used. Hydration reactivity of calcined magnesite at lower temperatures is much higher than that of magnesite calcined at 1100 degrees C, no matter which of three evaluation indexes is used (i.e., CT, X, and X-corrected).