An illumination system, a projection device, and a projection control method are provided. The illumination system includes a first light-emitting unit, a second light-emitting unit, a third light-emitting unit, a first dichroic element, a second dichroic element, and a control unit. The first light-emitting unit includes a first light-emitting element and a second light-emitting element. The control unit is electrically connected to the first light-emitting unit and configured to switch the illumination system between a high-performance mode and a high-chroma mode, wherein when the illumination system is in the high-performance mode, the control unit controls a current ratio of the second light-emitting element to be greater than a current ratio of the first light-emitting element, and when the illumination system is in the high-chroma mode, the control unit controls the current ratio of the second light-emitting element to be less than the current ratio of the first light-emitting element.