A three-dimensional display device and an active optical element thereof are provided. The three-dimensional display device includes a display panel, a polarizing element, and an active optical element between the display panel and the polarizing element. The active optical element includes a first substrate, a second substrate, a first electrode structure layer disposed on the first substrate, a second electrode structure layer disposed on the second substrate and a liquid crystal layer. The first electrode structure layer includes a plurality of first electrodes, a plurality of second electrodes alternately arranged with the first electrodes, and a first insulating layer located between the first electrodes and the second electrodes. The first electrodes and the second electrodes are extended along a first direction, and a first gap is formed between two adjacent second electrodes. An area of each first electrode fills one corresponding first gap.