An illumination system including a coherent light source, a light integrator and a first actuator is provided. The coherent light source is capable of providing an illumination beam. The light integrator is disposed on a transmission path of the illumination beam, and the first actuator is connected to the light integrator. The light integrator has a light entering end and a light exit end opposite to the light entering end, and the light entering end faces the coherent light source. The first actuator is capable of driving the light integrator to move and/or rotate, so as to change a position at the light entering end entered by the illumination beam.