The invention discloses an impurity in the preparation process of tadalafil. The impurity is a compound shown in a structural formula (2). The invention also discloses a preparation method of the compound shown in the structural formula (2), a purifying method of the tadalafil, and application of the compound shown in the structural formula (2). The impurity has the advantages that a novel impurity of tadalafil medicine is provided, and the preparation and the preparation method are put forward; the impurity is used as a standard product or reference product for the technology impurity of crude drug of tadalafil, so as to improve the quality control system of the crude drug of the tadalafil. The formula (2) is shown in the attached figure.