An experimental study has been performed on the desulfurization performance of the original magnesium slag (from the process of magnesium production). The total content of CaO and Ca2SiO4 is near 90% in the slag, but its conversion rate is below to 30% as desulfurizer directly in the experimental conditions. The aim of this paper is to enhance the performance of the slag by the hydration treatment process. In the hydration process, the slag, additives and water are mixed proportionally in slurry, then dehydrated and dried. Compared different slag performance by TGA, the optimal hydration method is found. The results show that a much higher calcium conversion rate (about 40%) of the slag is obtained after the hydration process. By means of XRD, Sorptomtic Instrument and SEM, the related parameters of the original slag and the hydration treated slag are analyzed. It is found that the composition, appearance and surface area changed obviously. The principle is basically studied that the hydration may enhance the desulfurization performance of the slag.