A dynamic mask adapted to an illumination system is provided. The illumination system includes at least one light source capable of emitting an illumination beam. The dynamic mask includes a board disposed in a transmission path of the illumination beam for adjusting a luminous flux of the illumination beam. The board has at least one strip-shaped opening. The strip-shaped opening has a first end and a second end opposite to the first end. The strip-shaped opening is mirror-symmetric with respect to a symmetrical axis, and the symmetrical axis is a straight line extending from the first end to the second end. A width of the strip-shaped opening increases from the first end to the second end, and the board is capable of moving along a direction parallel to the symmetrical axis. An illumination system using the dynamic mask is also provided.