The invention relates to a method for semi-continuous production of a fluorescent whitening agent and belongs to the field of production methods of fluorescent whitening agents. The method comprises the following steps: adding an acid binding agent at a uniform speed according to the molar ratio of cyanuric chloride to 4, 4-diaminostilbene-2, 2-disulfonic acid till the disappearance of an amino group to obtain 4, 4'-bis (4, 6-dichlorotriazine-2-amino) stilbene-2, 2'-disulfonate sodium; adding according to the molar ratio of 4, 4'-bis (4, 6-dichlorotriazine-2-amino) stilbene-2, 2'-disulfonate sodium to aniline, and reacting under the control of the acid binding agent and heating till the disappearance of the amino group to obtain a second-step condensation product for later use; and adding according to the molar ratio of diethanolamine to the second-step condensation product, further reacting an obtained third-step condensation product with the acid binding agent, then feeding into a pipeline reactor to obtain a reaction solution, and performing cooling, suction filtration, impurity removal, desalting and concentration to obtain a finished product. According to the method provided by the invention, the conversion rate of the reaction can be ensured, the product quality can be stabilized, the production efficiency can be improved, the productivity per unit time can be increased, the integrated production cost can be reduced, and the market competitiveness of an enterprise can be improved.