The invention relates to the technical field of organic synthesis, in particular to a preparation method for impurities in penehyclidine hydrochloride. The method comprises the steps that penehyclidine and alpha-phenyl-alpha-cyclopentyl-oxirane are adopted as raw materials, under the alkali compound effect, a reaction is performed to obtain 3-[2-cyclopentyl-2-phenyl-2-(2-cyclopentyl-2-hydroxyl-2-phenyl-ethyoxyl)ethyoxyl]quinuclidine free alkali, and after salt formation of hydrogen chloride, free alkali hydrochloride is obtained. The preparation method has the advantages that the technologicalsteps are simple, side reactions are few, operation is simple and convenient, post treatment is simple, raw materials are cheap and easy to obtain, and the yield is high, and the method is suitable for large-batch preparation.