Embodiments of the present invention disclose a depth of field maintaining apparatus, a 3D display system and a display method. The depth of field maintaining apparatus comprises: a 3D video signal input device; a position measuring device; an image processing device; and a 3D video signal output device, wherein the 3D video signal input device, the position measuring device and the 3D video signal output device are connected to the image processing device, the 3D video signal input device transmits an acquired 3D video signal to the image processing device, the position measuring device transmits detected position information of the viewer and the display screen to the image processing device, and the image processing device adjusts a space between corresponding object points in left and right eye images in a 3D video signal in accordance with the position information, such that a depth of field remains unchanged, and transmits a adjusted 3D video signal to the 3D video signal output device.