The invention relates to the field of chemical synthesis of drug, and relates to a method for preparing a rilpivirine hydrochloride isomer Z and impurity X and an application of the impurity X as an insecticide in agriculture. The isomer Z has a structure represented by a formula (I) in the specification, the impurity X has the structure shown by a formula (II) in the specification, and the rilpivirine hydrochloride isomer Z or the impurity X or a combination thereof is isolated from a reaction solution in the preparation of rilpivirine. The method improves the impurity spectrum of rilpivirinehydrochloride and has great significance for realizing the quality control of rilpivirine hydrochloride and its preparation.