Provided is an illumination system including an excitation light source, a first light splitting element, a first light reflecting element, a wavelength conversion device, and a light filtering device. The excitation light source provides an excitation beam including a first sub-beam and a second sub-beam. The first light splitting element includes a first zone and a second zone. The wavelength conversion device includes an optical zone and a wavelength conversion zone. In a first time period, the first sub-beam is transmitted to the optical zone via the first zone, and the second sub-beam is transmitted to the light filtering device via the first light splitting element and the first light reflecting element. In a second time period, the excitation beam is transmitted to the wavelength conversion zone to be converted into a first conversion beam. Therefore, the structure can be simplified and the size can be reduced.