Magnesium hydroxide sulfate hydrate (MHSH) whiskers were prepared using brine, sodium hydroxide and sodium sulfate as raw materials by one-step hydrothermal process. X-ray powder diffraction (XRD), thermal analysis (TG-DTG) and scanning electron microscope (SEM) were employed to characterize the MHSH whiskers. It was shown that the MHSH whiskers were MgSO4•5Mg(OH)2 •2H2O with a diameter of 0.2–0.3 μm and a length of 20–30 μm . The thermal decomposition of MHSH whisker was demonstrated a three-step scheme based on the TG-DTG data. The utilization ratio of magnesium in brine was above 80%.