A wavelength-converting wheel, for converting an excitation beam having an output power greater than or equal to 100 W into a conversion beam, is provided. The wavelength-converting wheel includes a turntable having an annular irradiation portion, an adhesive layer disposed at the annular irradiation portion, a reflective layer disposed on the adhesive layer, and a wavelength-converting layer. The wavelength-converting layer is disposed on the reflective layer and has a light receiving surface configured to be irradiated by the excitation beam. The invention further provides an illumination system including the wavelength-converting wheel, and a projection apparatus including the illumination system. The heat dissipating ability of the wavelength-converting wheel is improved. The material of the wavelength-converting layer is not deteriorated or damaged due to the heat of the excitation beam. The wavelength-converting layer has improved wavelength conversion efficiency and can effectively convert a high output power excitation beam into a conversion beam.