The invention provides a method for preparing a fulvic acid salt by activating lignite with a composite photocatalyst. The method comprises the following steps: evenly mixing lignite powder with water, then adding alkali to adjust the pH value to 9-11, carrying out stirring to react for 2-3 h at 45-75 DEG C, and carrying out solid-liquid separation to obtain a humate solution; adding acid, adjusting the pH value to 1-2, carrying out stirring for 10-20 min, standing for 10-30 min, and carrying out solid-liquid separation to obtain a solution and a precipitate; adding alkali into the solution for neutralization until the pH value is 7, and evaporating the solution to dryness to obtain a fulvic acid salt solid product; and adding the obtained precipitate into water, adding alkali to adjust the pH value to 8-10, then adding an oxidant and a composite photocatalyst, stirring the materials for 2-5 h under ultraviolet irradiation, carrying out solid-liquid separation, and evaporating the solution to dryness to obtain a fulvic acid salt solid product. The method provided by the invention has the advantages of simple production process, low cost, high humic acid activation rate and high fulvic acid salt yield, and is suitable for popularization and use in agricultural production.