The invention discloses a method for preparing 1-chloroanthraquinone through direct photocatalytic chlorination of chlorine gas. The method comprises the following steps: adding 1-nitroanthraquinone,a fluxing agent and a cocatalyst into a reaction container, and heating till a molten state; heating the chlorine gas, then enabling the chlorine gas to pass through an ultraviolet lamp irradiation chamber, and inflating the chlorine gas from the bottom into the reaction container; inflating nitrogen gas from the top of the reaction container, inflating tail gas into lye so as to be absorbed, stopping inflation of the chlorine gas and keeping inflation of the nitrogen gas when the content of the 1-nitroanthraquinone is lower than 0.5%, preserving heat, and reacting for 0.1-2 hours to obtain crude 1-chloroanthraquinone. The method has the beneficial effects as follows: through ultraviolet light catalysis, direct synthesis of the 1-chloroanthraquinone from the chlorine gas is catalyzed, so that the problems of the environment pollution caused by organic matter catalysis and the cost are avoided, the reaction temperature is reduced, the selectivity of the reaction is improved, the yield is increased, and zero discharge of heavy metals and zero discharge of waste residues during synthesis of the 1-chloroanthraquinone are achieved.