The invention relates to a method for preparing 2,4-dichloro-5-fluoropyrimidine through an ultraviolet photocatalytic reaction. The problem of environmental pollution caused by preparation and waste acid treatment of 2,4-dichloro-5-fluoropyrimidine can be effectively solved. The method comprises the steps that 2,4-dimethoxy-5-fluoropyrimidine is dissolved in a solvent the weight volume of which is10-20 times that of the 2,4-dimethoxy-5-fluoropyrimidine, wherein according to the weight volume, the unit of solids is gram, and the unit of liquid is milliliter; a catalyst sulfuryl chloride is added, and the addition amount of the catalyst sulfuryl chloride is 2-5% of the mass of the 2,4-dichloro-5-fluoropyrimidine; after ultraviolet light irradiation is conducted for 10 minutes, chlorine gasis introduced, the reaction temperature is 15-30 DEG C, and stirring while chlorine introduction is conducted for 12 hours; an equal volume of ice water is added, liquid separation is conducted, an organic phase is washed with an aqueous sodium carbonate solution with the mass concentration of 5%, and water washing, drying and concentration under reduced pressure are conducted to form the solid product 2,4-dichloro-5-fluoropyrimidine. The preparation method is simple, easy to operate, low in cost and free of waste residues, effectively prevents environmental pollution and has significant economic and social benefits.