The invention discloses a preparation method of 2,4-dichloro-5-fluorobenzoyl chloride and belongs to the field of organic synthesis. The preparation method comprises the following steps: with 2,4-dichlorofluorbenzene as a raw material, performing friedel-crafts reaction and hydrolyzation to generate an intermediate, 2,4-dichloro-5-fluorobenzoyl chloride; and hydrolyzing, oxidizing and acylating a byproduct, biopolymer (III), generated by the reaction to obtain a final compound (II), wherein the total yield is 88% or above. According to the invention, the raw material conversation ratio is 80% or above, the defects that existing raw materials are hardly available, and the utilization ratio is low are overcome, resources are saved, the production cost is lowered, and the preparation method is simple to operate and easy for amplified production.