High pure alumina was prepared using Bayer liquid. Gibbsite was treated by hydrothermal method under different conditions, then the impurity of sodium re-dissolved in water. Physical and chemical properties of the hydrothermal product were analyzed. The results showed that raising hydrothermal temperature to 180℃, gibbsite changed into boehmite. Na2O returned to dissolution in water during the transformation process of gibbsite, rising temperature and decreasing pH could promote the transformation, prolonging the reaction time could extend reaction, boehmite`s morphology, liking "broken glass" shape, was hugely different from gibbsite after the hydrothermal treatment. Compared with gibbsite, the boehmite was looser and more unordered. Under the condition of pH=1, the content of Na2O in hydrothermal production was as low as 0.0163wt%, which can reduce to 0.0187wt% after calcination at 1200℃.