An active grating, a three-dimensional display device and a three-dimensional display method are provided. The three-dimensional display device comprises: a display panel (1) and an active grating (2) disposed oppositely; a face recognition module, configured to acquire a distance from a current viewer to the active grating (2); a first calculating module, in signal connection with the face recognition module, configured to calculate a theoretical horizontal width of each of the light shielding regions (21) when the current viewer see an ideal 3D image at a current position, according to the distance acquired by the face recognition module and a first corresponding relationship between the distance from the current viewer to the active grating and the theoretical horizontal width of each of the light shielding region of the active grating when the current viewer see the ideal 3D image; and an adjusting module, in signal connection with the first calculating module and a driving circuit of the active grating respectively, configured to send a corresponding first control instruction to the driving circuit according to the theoretical horizontal width calculated by the first calculating module so as to adjust an actual horizontal width of each of the light shielding regions (21) of the active grating, so that the actual horizontal width equals to the theoretical horizontal width. The three-dimensional display device provided by embodiments of the invention can change the viewing region position according to position coordinates of the viewer, so that the ideal 3D image can be seen by the viewer not positioned at a fixed viewing region position, thus, improving practicality of the three-dimensional display device obviously.