The invention relates to an energy-saving method in production of aluminium oxide and especially relates to a high-pressure dissolution-out and evaporation combined energy-saving method in production of the aluminium oxide. The method includes following steps: heating dissolved-out raw ore pulp with new steam employed in a high-pressure dissolution-out process to obtain new steam condensed water; performing a flash process to the new steam condensed water to form a flash steam under a pressure of 0.4-0.6 MPa; sending the flash steam through pipelines to a mother liquid evaporation process for partially replacing new steam; carrying out a multistage flash process to the dissolved-out raw ore pulp after the high-pressure dissolution-out process, wherein a flash steam, under a pressure of 0.4-1.0 MPa, is generated in first-stage flash from the fourth stage to the sixth stage in the multistage flash process with from the ninth stage to the eleventh stage being pre-heated; and sending the flash steam to the mother liquid evaporation process through pipelines for partially replacing the new steam. The method is characterized by employing a new high-pressure dissolution-out and evaporation combined energy-saving technology so that thermal energy in a system can be utilized better, thereby achieving a purpose of saving energy.