Reflection type diffusion device and projection device

The invention relates to a reflective diffusion device, which comprises a reflecting piece and a diffusion piece, and is used for reflecting and diffusing a laser beam emitted by a light source module. The reflector has a central axis and includes a reflecting surface facing the light source module. An included angle larger than 0 degree and smaller than 90 degrees is formed between the reflecting face and the central axis. The diffusion piece is arranged on at least one part of the reflecting surface of the reflecting piece. A laser beam enters a first position of the reflective diffusion device in an incidence direction parallel to the central axis, is reflected by the reflecting surface, is transmitted and enters a second position of the reflective diffusion device, and is reflected by the reflecting surface to maintain the laser beam or form a diffused beam. The laser beam or the diffusion beam is emitted from the reflective diffusion device in an emitting direction parallel to the central axis, and the f