Formation,solution and phase change of hydration products in MgO-MgCl_2-H_2O system was studied with thermodynamics method,and resistance to water immersion and phase change of magnesium oxychloride cement with different MgO/MgCl_2 molar ratio was experimented. The results show that pH value of immersion solution of cement paste has a remarkable influence on phase stability of hydration products.A higher pH value leads to a lower solubility and a better phase stability of hydration products.When the solution pH value is higher than 10.37,the precipitation of much Mg(OH)_2 crystal induces a worse phase stability of hydration products.With the increasing MgO/MgCl_2 molar ratio(lower than 6),the more amount of MgO in the hydration products enhances the alkalinity of solution and the phase stability is improved.However,when the MgO/MgCl_2 molar ratio is higher than 6 and the excessive MgO exsits in the hydration products,the cement paste may be damaged by the excessive crystallization stress of a great deal of Mg(OH)_2 formation.