A device for converting wavelength and a projection light source are disclosed in the present disclosure. According to an example, the device may comprise a roller, a drive motor, a first reflector, a support shaft, a drive module and a dichroic film. An axis of the support shaft may be coincided with an axis of the roller. The support shaft may be not rotated with rotation of the roller. The drive motor may drive the roller rotate around the support shaft and the drive module may drive the roller to move back and forth along an axial direction of the support shaft. So excitation light reflected by the first reflector may be incident uniformly on the sidewall of the roller. Therefore, fluorescent powder conversion efficiency may be improved compared with that of the excitation light incident on a fixed area.