The disclosure relates to an illumination system, a projection device and an illumination control method. The illumination system includes a first sensor, a second sensor and a control module. The first sensor receives a part of an excitation beam and a part of at least one converted beam scattered by a wavelength conversion module, so as to generate a first photoelectric signal. The second sensor receives a part of a first set of color light and a part of a second set of color light scattered by a filter module, so as to generate a second photoelectric signal. The control module generates a synchronization signal based on relative intensity changes of the first and second photoelectric signals. The synchronization signal is to synchronize the wavelength conversion module with the filter module, and the first set of color light and the second set of color light sequentially form an illumination beam.