Magnesium oxychloride cement (MOC) has been investigated by many researchers who have studied its hydration products and properties. Previous works mainly concentrated on experimental studies. This study uses a thermodynamic approach to understand the formation conditions of two major hydration products of MOC, Mg(OH)Cl·4HO (phase 5) and Mg(OH)Cl·4HO (phase 3) at room temperature. The hydration reaction equilibrium of MOC phases has been predicted by using the geochemistry speciation code PHREEQC together with the extended 'Pitzer.dat' database. The results show that the formation of the hydration products are controlled by MgCl concentration, activity of HO and pH values of the system. The equilibrium of solid phase diagram, the solubility of different hydration phases are consistent with those results obtained from experimental studies, which validates the thermodynamic model. The phase diagram provides qualitative insights on the synthesis of pure phase 3 and phase 5. For MgO-MgCl-HO system, the minimum MgCl concentration for phase 5 and phase 3 to form are 1.47 mol·kg and 2.25 mol·kg, respectively. Increasing water volume can result in the transformation from phase 5 and phase 3 to brucite. Additionally, the stability analysis suggests that phase 5 prefers to formation at a higher condition of the a2 +, pH and a as compared to the phase 3. Phosphate can significantly influence the hydration products composition of MOC during hydration but no new phosphate appears.