An illumination system includes a coherent light source providing an illumination beam, a light integrator positioned on a transmission path of the illumination beam, and a first actuator 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 about a rotation axis perpendicular to a light axis of the illumination beam passing through the light integrator, so as to change a position at the light entering end entered by the illumination beam.