Magnesium hydrate is the absorbent in magnesia scrubbing wet flue gas desulfurization. Its dissolution rate has a great impact on the performance of SO2 removal. Further, magnesia is the heat decomposition product of magnesite. The effect of magnesite heat decomposition temperature on the dissolution rate of magnesium hydrate is investigated in this study. As can be seen from the experimental results, magnesium hydrate dissolution rate increased with increasing heat decomposition temperature up to 700 degrees C and then decreased with the increase of heat decomposition temperature. This is caused by the change of magnesite heat decomposition mechanism at a different heat decomposition temperature.