A diffraction-type 3D display element is arranged on an image output face of a 3D display device and comprises a first diffraction area and a second diffraction area. The first diffraction area has a plurality of first stepped gratings spaced apart from each other. The second diffraction area has a plurality of second stepped gratings spaced apart from each other. The second diffraction area is adjacent to the first diffraction area and is arranged symmetrically to the first diffraction area with a central line being the symmetric axis. The diffraction-type 3D display element of the invention diffracts the images output by the 3D display device and projects the diffracted images to two different viewing areas to provide 3D images for users.