The invention belongs to the technical field of medicament synthesis, and in particular relates to a preparation method of an olmesartan intermediate. The preparation method comprises the following steps: enabling a compound (III) to react with a strong base in the presence of an organic solvent under the protection of nitrogen to generate a compound (II), enabling the compound (II) to react with 4-chloromethyl-5-methyl-1,3-dioxole-2-one under the catalysis of potassium carbonate and potassium iodide without separation, performing extraction with an organic solvent after reaction, and performing brine washing, concentration, crystallization, centrifuging and drying to obtain a crude product of a compound (I); and recrystallizing the crude product of the compound (I) by using the organic solvent, then centrifuging, and finally performing vacuum drying under the condition of alternate replacement of nitrogen and vacuum, thereby obtaining a high-quality product of the compound (I). The preparation method provided by the invention is stable in process and simple in operation, the process condition is easy to control, the after treatment is simple and convenient, less three wastes are generated, a mother solution can be continuously and circularly used, the product quality is good, the yield is high, the production cost is low, and a pilotscale experiment validates that the preparation method is suitable for industrial production.