A projection system includes a light source, a projection lens, and a light-adjusting diaphragm element. The light source emits a first light having a first dominant wavelength range and a second dominant wavelength range. The projection lens is disposed on the path of the first light. The light-adjusting diaphragm element, disposed between the light source and the projection lens, has an opening and a light shielding area outside the opening, a filter is disposed in the opening, wherein a part of the first light is blocked by the light shielding area, a wavelength band corresponding to the first dominant wavelength range of other part of the first light passes through the opening, and a wavelength band corresponding to the second dominant wavelength range of the other part of the first light is blocked by the filter to become a second light having a narrow wavelength range.