An optical system including a light source, a filtering module, a wavelength conversion unit, and a reflection unit is provided. The light source emits an illumination beam. The filtering module is disposed on a transmission path of the illumination beam and includes a plurality of filtering units having different colors. The filtering units move into the transmission path of the illumination beam in turn. Each filtering unit reflects a part of the illumination beam and allows another part of the illumination beam to pass through. The wavelength conversion unit is disposed on a transmission path of the part of the illumination beam reflected by at least one of the filtering units to convert the part of the illumination beam reflected by the at least one of the filtering units into a converted beam. The wavelength conversion unit is disposed on the reflection unit.