An illumination system configured to be included in a projection apparatus comprising a first light valve and a second light valve, the illumination system comprising: a first excitation light source configured to emit a first excitation beam; a wavelength conversion element having a first region and a second region and comprising a wavelength conversion material in the first region and in the second region, and a driving element to drive the wavelength conversion element to rotate around a rotation shaft so as to rotate the first region and the second region to sequentially enter into a transmission path of the first excitation beam, wherein a concentration of the wavelength conversion material in the first region is greater than a concentration of the wavelength conversion material in the second region, wherein the first excitation beam is transmitted to the first region in a first time interval, the first excitation beam is configured to excite the wavelength conversion material in the first region to emit a first conversion beam, and the first excitation beam is transmitted to the second region in a second time interval, a part of the first excitation beam excites the wavelength conversion material in the second region to emit a second conversion beam, and a different part of the first excitation beam passes through the second region; wherein the first time interval and the second time interval are two consecutive time intervals; and a scattering element is disposed on the transmission path of the first excitation beam to be rotated with the wavelength conversion element synchronously, wherein the illumination system outputs the first conversion beam, a part of the second conversion beam, and the different part of the first excitation beam passing through the second region to the first light valve and the second light valve. a first excitation light source configured to emit a first excitation beam; a wavelength conversion element having a first region and a second region and comprising a wavelength conversion material in the first region and in the second region, and a driving element to drive the wavelength conversion element to rotate around a rotation shaft so as to rotate the first region and the second region to sequentially enter into a transmission path of the first excitation beam, wherein a concentration of the wavelength conversion material in the first region is greater than a concentration of the wavelength conversion material in the second region, wherein the first excitation beam is transmitted to the first region in a first time interval, the first excitation beam is configured to excite the wavelength conversion material in the first region to emit a first conversion beam, and the first excitation beam is transmitted to the second region in a second time interval, a part of the first excitation beam excites the wavelength conversion material in the second region to emit a second conversion beam, and a different part of the first excitation beam passes through the second region; wherein the first time interval and the second time interval are two consecutive time intervals; and a scattering element is disposed on the transmission path of the first excitation beam to be rotated with the wavelength conversion element synchronously, wherein the illumination system outputs the first conversion beam, a part of the second conversion beam, and the different part of the first excitation beam passing through the second region to the first light valve and the second light valve.