The invention discloses a preparing method for a cyclohexyl impurity of ACEI medicine. The preparing method comprises the following steps that firstly, raw ACEI medicine is dissolved in alcohol, then glacial acetic acid and a catalyst are added, and hydrogen is introduced after nitrogen displacement is carried out three times, wherein the pressure of hydrogen is controlled to be 10-50 psi, the reaction temperature ranges from 20 DEG C to 50 DEG C, and the reaction time ranges from 6 h to 48 h; secondly, a reaction solution obtained in the first step is filtered, filter liquor is subjected to vacuum concentration with the concentration temperature not higher than 35 DEG C, then concentrate is separated and purified with HPLC, purified substances continue to be concentrated and dried, and the cyclohexyl impurity of ACEI medicine is obtained. The preparing process is low in cost, operation is easy, and a qualified and cheap comparison product easy to obtain can be provided for quality control over ACEI medicine.