The invention relates to a method for preparing sodium sulfate from baking soda desulfurized fly ash by a wet process, which comprises the following steps: firstly detecting the content of each component in the baking soda desulfurized fly ash, mixing with water to prepare turbid liquid after the content is qualified, then sequentially adding an oxidizing agent and an acid reagent for reaction, and finally evaporating, crystallizing and separating to obtain a target product Na2SO4. 10H2O. By means of the method, the steel mill solid waste baking soda desulfurization ash can be thoroughly converted into a high-purity and high-economic-value sodium sulfate hydrate product, the product is widely applied to multiple industries such as chemical engineering, food and pharmacy, and good economic benefits can be generated. Compared with existing other desulfurization ash solid waste resource recycling and reusing methods, the method has the advantages of being large in treatment capacity, thorough in conversion, high in product purity, little in waste water, free of limitation of other conditions and the like, and construction of green steel mills is promoted.