An illumination system, a projection device, and an illumination method are provided. The illumination system includes a polarized light source, a polarization switching element, a beam splitting element, a wavelength conversion element, a reflective element, and a wave plate. The polarized light source emits a polarized light beam with a first color. The polarization switching element switches a polarized direction of the polarized light beam at different time points. The beam splitting element separates polarized light beams having different polarized directions. The wavelength conversion element converts the polarized light beam having a first polarized direction to a converted light beam. The reflective element reflects the polarized light beam having a second polarized direction to form a reflected light beam. The beam splitting element combines the converted light beam and the reflected light beam comes from the wave plate disposed between the beam splitting element and the reflective element.