Based on the inherent line-element hologram of rainbow holography, this paper presents the illustration of the imaging mechanism of rainbow holography by no means of the traditional concept of slit images. The relationship between the width of line-element hologram and the minimum resolved distance as well as the stereo limit of the reconstructed image are discussed in detail. Characteristics of the line-element holograms and their effects on several typical rainbow holograms are illustrated and a new approach of observing the reconstructed image without the real slit image is proposed.