Magnesium oxychloride (MOC) cement is featured with high early strength, low thermal conductivity and low density, but is not widely applied in construction engineering due to its poor water resistance capability. This research has studied the effect of phosphoric acid and tartaric acid additions on the water resistance of MOC cement pastes, in which also reports the effects on setting time, hydration reactions, compressive strength, phase composition, thermal stability and microstructure. 1 wt% of phosphoric acid and tartaric acid additions can improve the water resistance and reduce thermal stability of MOC cement pastes, which is associated with formation of gel-like 5Mg(OH) 2 ·MgCl 2 ·8H 2 O. Moreover, these additions reduce the compressive strength and prolong the setting time of MOC cement pastes, as well as increase the total porosity, the volume fraction of gel pores (<10 nm) and large pores (>100 nm), however, decrease the volume fraction of small capillary pores (10–100 nm) of MOC cement pastes. These effects are caused by both additives but are most pronounced for MOC cement pastes containing phosphoric acid. In addition, 2Mg(OH) 2 ·MgCl 2 ·2H 2 O is a transitional phase in the formative stage of 5Mg(OH) 2 ·MgCl 2 ·8H 2 O in MOC cement.