Doumu, the Mother of the Dipper, is a Daoist female deity, but she has an unusual image among all of the Daoist female deities, for she often appears as an esoteric Budhisattva who has three heads and six or eight arms. This paper examines the extant image materials of Doumu of the Ming and Qing dynasties and generalizes their traits in different periods, on one hand, and analyzes the distinct contents of related scriptures of Doumu, on the other. It demonstrates that the esoteric image of Doumu derived from Marici, a goddess of light in Esoteric Buddhism, as Doumu was absorbed into the canons of Thunder Rites within which her name combined with Marici's, her magic assimilated Marici's conjuration, and at the same time, she borrowed Marici's image. Since then, Doumu has not only been the mother of the Dipper, but the main deity of Thunder Rites, and her image has changed from a traditional Daoist female deity into an esoteric Budhisattva. As the characteristics and functions of Doumu were different from Marici, the borrowing image of Doumu adjusted the most prominent feature to her identity and her image thence has specific features of her own. Besides the investigation of image materials and scriptures of Doumu, this paper discusses the meaning of the distinguishing feature of her borrowing image and the main factors prompted this image borrowing.