The invention relates to a method used for avoiding sewage penetration in aluminum oxide production processes. The method comprises following steps: planes of a plurality of layers of a double-pipe heat exchanger are provided with cofferdams, so that washing water is collected when casing pipes are cleaned with water, and the collected washing water collected from the planes is delivered into a water collection tank; the water collection tank is provided with a sedimentation basin, and residue obtained by sedimentation of the sedimentation basin is removed periodically; overflow of the sedimentation basin is delivered into the water collection tank, is delivered into a heat water groove arranged at a sedimentation system, and is heated and is used as red mud washing water. Taking equipment provided with 70 of single set dissolution casing pipes as an example, advantages and effects of the method are that: more than 300 cubic meters washing water can be recycled in each washing period, and if evaporation steam/water ratio is 0.26, then 78t of steam consumed because of sewage penetration into processes can be reduced by washing water recycling in each washing period.