The invention provides a method for preparing fulvic acid salt by using a composite photocatalyst to activate low-rank coal. The method comprises the following steps of uniformly mixing the low-rank coal powder and water, adding alkaline to adjust the pH (potential of hydrogen) value to 8 to 11, stirring for 1 to 2h, adding an oxidant, reacting for 1 to 3h at the temperature of 25 to 55 DEG C, andseparating solid and liquid, so as to obtain a humic acid salt solution; adding the composite photocatalyst, stirring to react for 2 to 6h under the condition of ultraviolet radiation, separating solid and liquid, taking the solution and evaporating, so as to obtain the solid fulvic acid salt product. The method has the advantages that the production technology is simple, the green and environment-friendly effects are realized, the production cost is low, the activating rate of the humic acid in the low-rank coal is high, and the yield rate of the fulvic acid salt prepared by the low-rank coal is obviously improved.