An illumination system includes an exciting light source, a first lens set, a dichroic element, a wavelength-converting element and a beam splitter. The exciting light source emits an excitation beam. The dichroic element reflects the excitation beam to the first lens set. The first lens set has an optical axis. The excitation beam is incident into the first lens set in a direction parallel to the optical axis and passes through the first lens set. The beam axis of the excitation beam is noncoaxial with the optical axis. The wavelength-converting element has a reflective region and a wavelength-converting region. The wavelength-converting region converts the excitation beam into a converted beam, which passes through the dichroic element and the beam splitter. The reflective region reflects the excitation beam to the first lens set and the beam splitter. The beam splitter splits the excitation beam into two branch excitation beams.