A projection system, a light source system and a light source assembly. The light source system includes an excitation light source, a wavelength conversion device, a filter device, a drive device and a first optical assembly. The wavelength conversion device includes at least one wavelength conversion region; the filter device is fixed relative to the wavelength conversion device and includes at least one first filter region. The drive device drives the wavelength conversion device and the filter device, so that the wavelength conversion region and the first filter region synchronously move, and the wavelength conversion region is periodically disposed on a propagation path of excitation light, to wavelength-convert the excitation light into the converted light. The first optical assembly guides the converted light into the first filter region; and the first filter region filters the converted light to improve its color purity.