Magnesium oxychloride cement was prepared by the reaction between light-burned MgO and MgCl₂·6H₂O in presence of citric acid and ammonium citrate tribasic. Effects of citric acid and ammonium citrate tribasic on improving the water stability of magnesium oxychloride cement and its mechanism were studied. The results show that the citrate combines with MgOH⁺ to form a stable complex， hindering the precipitation of Mg（OH）₂ and promoting the generation of 5Mg（OH）₂·MgCl₂·8H₂O （5-1-8 phase）， which improves the compressive strength and volume stability of magnesium oxychloride cement at all ages. The addition of citric acid and ammonium citrate tribasic induces none of new phase， but significantly enhances the water stability of magnesium oxychloride cement due to the adsorption of citrate onto the surface of 5-1-8 phase. The improvement of water stability by citric acid is better than that by ammonium citrate tribasic.