Magnesium sulfate (MgSO4) is the most widely used protector for alleviating the effects that metal ions have on cellulose degradation. However, the efficiency of MgSO4 is limited by the oxygen delignification conditions. This work discusses the factors influencing MgSO4 efficiency in terms of cellulose protection and delignification. The type and concentration of metal ions, delignification rate, additions order, and mixing degree of MgSO4 should affect the cellulose degradation during oxygen delignification in the presence of MgSO4. The most adverse effects on cellulose are observed with Mn2+ and Fe2+ ions followed by Cu2+ and Fe3+. MgSO4 addition can diminish such negative effects; however the protection becomes reduced in the presence of higher concentrations of metal ions. In addition, the optimum MgSO4 application level is closely dependent on the delignification rate and metal ions concentration. Adding MgSO4 is optional for pulps with trace metal ions at relatively low delignification levels, but it is essential for pulps with concentrated metal ions or when the oxygen delignification rate is relatively high. More simply, when the added MgSO4 is thoroughly mixed with the pulp before the addition of NaOH, it exhibits a prominent effect on cellulose protection.