Photobiomodulation (PBM) as an adjunct method for the treatment of oral mucosal diseases has developed rapidly. It is through the absorption of photon energy by cells, produce photochemical effects, and thus regulate a variety of biological processes to achieve therapeutic purposes. In this paper, the four aspects of reducing inflammation, accelerating tissue healing, relieving pain and bidirectional dose of PBM are reviewed, and the mechanism of their action is deeply discussed in order to provide clinicians with better clinical decision and basis for the treatment of oral mucosal diseases with PBM therapy.