By using aluminum sulfate as precursor and urea as precipitator，hollow sphere boehmite was synthesized by hydrothermal treatment at 140℃ for 24 h. The time effect on the as-synthesized product was studied by XRD，SEM and TG. The results showed that with increasing hydrothermal treatment time the product’s phase was transformed from basic aluminum sulfate to boehmite. After 24 h hydrothermal treatment，the product could be completely changed to boehmite. With increasing hydrothermal treatment time，the micromorphology of the hydrolysis product experienced the evolution of smooth dense globe to core-shell sphere to hollow sphere.