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A series of quenching hydration experiments of magnesium slag were carried out under different conditions (quenching and hydration temperatures, hydration time, liquid/solid ratio, and continuous/non-continuous process). During the hydration process, the electrical conductivity, pH, and hydration degree were obtained to characterize the reaction. Combined with the hydration degree, kinetic models were used to analyze the hydration kinetics and obtain the relevant parameters. To further understand the desulfurization performance of the hydrated magnesium slag, and the relationship between hydration and desulfurization, desulfurization experiments were carried out in a thermal gravimetric analyzer. Further, changes in the material composition and morphology after hydration were investigated by X-ray diffraction and scanning electron microscopy, respectively. The results showed that the non-continuous hydration could be described by a modified Krstulovic-Dabic model. The highest hydration degree (0.16) was achieved at the quenching temperature of 950 °C and hydration temperature of 80 °C. Orthogonal tests established the optimum conditions for continuous hydration: quenching temperature of 950 °C, liquid/solid ratio of 8, and hydration time of 8 h. In decreasing order, these three factors are ranked as hydration time > quenching temperature > liquid/solid ratio. The desulfurization performance of samples treated with continuous hydration was better than that treated with non-continuous hydration, with the calcium conversion rates of 30.3% and 13.3%, respectively.