The invention discloses a new impurity related to gefitinib, namely 3-(3-chloro-4-fluoroanilino)-7-methoxy-6-(3-morpholinyl propyl) quinazoline-4 (3H)-one, as well as a preparation method thereof. The impurity is generated in the processes of reacting initial raw material 2-amino-4-methoxy-5-(3-morpholinyl propyl) cyanophenyl with N,N-dimethyl formamide dimethyl acetal to obtain Schiff base and then implementing heating annulation with 3-chloro-4-fluoroaniline in an acetic acid solvent. The new impurity is novel in structure, and is favorable for quality control of a gefitinib crude medicine. Meanwhile, the invention further discloses a preparation method of the impurity, comprising a mother liquor recovery column chromatography method and an oriented synthesis method. The method has the characteristics of short synthesis route, simple operation, relatively high product purity and is applicable to study on reference substance and the like.