Decomposition of sodium aluminate solution is the key step for alumina production; however, approximately only 55% alumina yield is obtained by seeded precipitation. So, it is difficult to obtain sodium aluminate solution with high caustic ratio. In this study, a method was explored to recover alumina from spent Bayer liquor by deep decomposition with methanol. A variety of conditions, including reaction temperature, reaction time, methanol amount and seed coefficient were elaborately investigated. The results showed that the appropriate conditions were 1:1 in volume ratio of methanol to spent Bayer liquor, more than 1.0 seed coefficient and in a 40 ℃ water bath for 24 h. By characterizing through XRD, the crystal products were found to be Al(OH)3. With this method, the molar ratio of Na2O to Al2O3 of the spent liquor can be increased from about 3.0 to 10.0 due to the recovery of alumina, which is beneficial for the treatment of red mud.