The invention relates to a preparation method of porous Al2O3 ceramic. The preparation method comprises the steps: adopting sintered-plate shaped alundum powder as raw material, adopting water glass solution as a binding agent, mixing the sintered-plate shaped alundum powder and the water glass solution into suspending liquid, fully stirring, injecting the obtained suspending liquid into a mold, freezing the suspending liquid in a vacuum freeze dryer, carrying out continuous drying thoroughly under vacuum state on the frozen suspending liquid, then demolding the sample, placing the demolded sample into a high-temperature box-type electric furnace, rising the temperature to 1500-1600 DEG C, keeping the temperature for 2 hours, and cooling naturally along the furnace to obtain the porous Al2O3 ceramic. As a vacuum freeze drying technique is adopted, the invention does not add any organic or inorganic chemical harmful substances, does not need special templates, and adopts ice as a hole-making template; and after the ice crystal is sublimated and removed, the ceramic particles obtain a porous microstructure of negative duplicates for growth of the ice crystal. The preparation method better inherits the ice crystal appearance, also does not need the processes of removing the templates, such as calcining, chemical etching and the like, not only has simple technique, but also has unique hole structure, and is easy to obtain a special hole structure.