We have developed a facile MgO‐based wet oxidation process for biomass pretreatment. Herein, we performed kilo‐scale experiments to elucidate the transformations of Mg in the process as well as the recycling of the MgO‐based solid alkali. A new intermediate, magnesium oxide carbonate [Mg₃O(CO₃)₂] generated in situ during the process was shown for the first time, which could provide an indispensably weak but adequate alkaline environment to support the oxygen delignification reaction. However, once the solid alkali was transformed into hydromagnesite [Mg₅(CO₃)₄(OH)₂] with sufficient CO₂, the hydromagnesite form would prevent the solid alkali from developing resistance to the acidic gaseous CO₂. Ultimately, if we take the features of the solid alkali into consideration, alkali recovery for this process was fulfilled easily without concentration or causticization, which are of high cost but unavoidable in the traditional recycling of sodium alkali.