An illumination system including light source devices, a light receiver, a calculation module and a control module is provided. The light source devices emit light beams having different frequencies respectively. The light receiver receives at least one of the light beams emitted from the light source devices. The calculation module is coupled to the light receiver and obtains at least one optical parameter of the at least one of the light beams according to the at least one of the light beams received by the light receiver. Each of the at least one optical parameter includes a light intensity, a color temperature, a color rendering index or an illumination ratio. The control module is coupled to the calculation module and the light source devices. The control module controls the at least one optical parameter of the at least one of the light beams. A control method thereof is also provided.