Magnesium oxysulfate (MgSO4·5Mg(OH)2·2H2O) flake powders with an average diameter of 2 μm and a thickness of 0.052 μm were prepared using magnesium sulfate and sodium hydroxide as raw materials by hydrothermal synthesis process.The composition,morphology and structural features of the hydrothermal products were examined with X-ray powder diffraction (XRD),scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR).The experimental results indicate that in the conditions of n(NaOH)/n(MgSO4) of 1.25,the dosage of w(Na3PO4) crystal additives of 1.0％ w(MgSO4),stirring for 5 h at 180 ℃,the morphology of MgSO4·5Mg(OH)2·2H2O products is flaky and laminar,which is a kind of complex magnesium singlecrystal.The recycling of MgSO4 mother liquor was also investigated to make a full use of the materials and reduce disposal.The results prove that there is no adverse effect on the yield and purity of the products.