A projection system including an illumination unit, a polarization conversion unit, a light valve, a projection lens, and a polarizing beam split (PBS) element is provided. The polarization conversion unit includes a V-shaped PBS element, a wave plate, and two reflective surfaces. The V-shaped PBS element includes a first PBS portion and a second PBS portion. The first PBS portion is adapted to be passed through by a first partial beam with a first polarization direction and reflect a second partial beam with a second polarization direction. The second PBS portion is adapted to be passed through by a third partial beam with the first polarization direction and reflect a fourth partial beam with the second polarization direction. The wave plate is disposed in the transmission paths of the first partial beam and the third partial beam. The reflective surfaces are located at opposite sides of the V-shaped PBS element.