Objective To study the role of photobiomodulation(PBM)in promoting the repair of spinal cord injury(SCI)by regulating microglial cells.Methods Forty-five C57BL/6J mice were randomly divided into the sham operation(Sham)group,surgery(SCI)group and the treatment(SCI+PBM)group,with 15 mice in each.After laminectomy of the T10 vertebral body in the three groups of mice,the SCI group and the SCI+PBM group were used to construct the model of spinal cord hemisection.The SCI+PBM group received immediate PBM treatment after spinal cord injury,while the other two groups did not.On the 1st,3rd,7th,14th,21st and 28th days(D1,D3,D7,D14,D21,D28)after the operation,the Basso Mouse Scale(BMS)was used to assess the recovery of the hind limb motor function of the mice.On the 28th day post operatively,immunofluorescence was used to detect the changes of neurons in the areas of injury in the three groups of mice.Quantitative real-time PCR and Western blotting experiments were used to detect the phenotypic changes of BV2 cells under the interventions of PBM with inflammatory stimulation.Western blotting experiments were conducted to detect the effects of PBM on the nuclear factor kappa-B(NF-κB)pathway.Results On the 28th day after the operation,the results of the mouse motor assessment showed that the BMS scores and related behaviors of the mice in the SCI+PBM group were better than those of the mice in the SCI group(P＜0.05),and the neurons in the SCI+PBM group far outnumbered those in the SCI group(P＜0.05).The results of quantitative real-time PCR and Western blotting experiments showed that on the 14th day after the operation,PBM promoted the activation of M2-type microglial cells in vivo but inhibited the activation of M1-type microglial cells.In vitro experiments confirmed that PBM could promote the polarization of BV2 cells towards M2-type microglial cells.In addition,PBM inhibited the activation of the NF-κB pathway in injured spinal cords and in activated BV2 cells.Conclusion PBM can promote the repair of spinal cord injury in SCI mice by promoting microglial cells through inhibiting the NF-κB pathway.