A projection system including a projection screen and a projection apparatus is provided. The projection screen has a plurality of optical microstructures. The optical microstructures are arranged along a first direction. The projection apparatus is capable of moving relative to the projection screen along the first direction. The projection apparatus includes an illumination unit, a light valve, and a projection lens. The illumination unit is capable of providing an illuminating beam. The light valve is disposed on a transmission path of the illuminating beam. The light valve is capable of converting the illuminating beam to an image beam. The projection lens is disposed on a transmission path of the image beam. The projection lens is capable of moving relative to the light valve along a second direction and projecting the image beam on the projection screen. The first direction and the second direction are opposite directions.