A three dimensional (3D) display device and a display method thereof are provided. The 3D display device includes a two dimensional (2D) display panel (100) and a slit grating (200) superimposed on the 2D display panel (100). The 2D display panel includes a plurality of subpixels sequentially arranged in a row direction and a column direction; the slit grating includes a plurality of light-transmitting strips (202) and a plurality of light-shielding strips (201) which are parallel to each other and alternately and periodically arranged. An angle formed by a central axis of each of the light-transmitting strips or a central axis of each of the light-shielding strips and the column direction is an acute angle. An area between the central axes of two adjacent light-shielding strips is divided into a first subarea disposed on a first side and a second subarea disposed on a second side by a central axis of a light-transmitting strip between the two adjacent light-shielding strips; subpixels whose area falling into the first subarea is greater than ½ subpixel area are first viewpoint subpixels; and subpixels whose area falling into the second subarea is greater than ½ subpixel area are second viewpoint subpixels.