The invention discloses a large-size AlOH ceramic low-temperature preparing method. The method includes the steps of conducting anti-hydration treatment on AlOH ceramic powder, preparing water-base slurry for casting, conducting cast-molding and the like. The limitation problems can be well solved that the powder flowability is poor, a forming biscuit is low in density and the device cost requirement is high in the large-size ceramic mold-pressing forming preparation process. By designing a proper mold and selecting proper gel, the forming preparation of special-shaped ceramic with the large size, large thickness and complicated structure can be realized; the selected gel system can have gel conversion at the room temperature compared with a high-glycan gel system, there is no need to induce the conversion through heating, and operation is simpler and easier.