The invention relates to the field of fluorescence emission, and particularly discloses a method for preparing a manganese complex blue fluorescent material in a dimer form. The method comprises the following preparation steps: dissolving 1,6-naphthalenedisulfonic acid, MnCl2.4H2O and 1,10-phenanthroline into 25 milliliters of an aqueous solution of methanol; transferring a solution obtained in the previous step into a 30-milliliter stainless steel reaction kettle lined with polytetrafluoroethylene, reacting for 70 to 80h under the constant temperature condition of 120 DEG C, slowly cooling the solution to room temperature, filtering to obtain yellow bulk crystals, washing the product with de-ionized water, and naturally drying the product in air. The obtained product has high thermal stability; no rare earth elements but transition metal manganese is adopted for synthesis of the complex; the synthesis method is simple, and comprises fewer synthesis steps, the synthesis cost is low, and the adopted raw materials are low-cost; the complex has high fluorescence emission performance.