Abstract: An illumination system includes an excitation light source, a first variable focus lens, a second variable focus lens, and a control unit. The excitation light source is adapted to provide an excitation light beam. The first variable focus lens is disposed on a transmission path of the excitation light beam. The first variable focus lens is located between the excitation light source and the second variable focus lens. The second variable focus lens is adapted to receive the excitation light beam exited from the first variable focus lens. The control unit is electrically connected to the first variable focus lens and the second variable focus lens, and is adapted to synchronously adjust focal lengths of the first variable focus lens and the second variable focus lens. A projection device including the above-mentioned illumination system of the invention is further provided.