Objective To study the type variation of microglial activation in spinal dorsal horn of rats after sciatic nerve injury. Methods Healthy adult male Sprague-Dawley rats were randomly divided into the control and experimental groups, 24 rats in each group. The experimental group underwent ligation of sciatic nerve trunk to generate nerve injury in the rats. The pain behavior in the rats was measured at the 1th, 7th and 14th postoperative days, and the changes of microglial activation in the rat lumbar spinal cord dorsal horn was detected by immunofluorescence staining. qRT-PCR assay was used to validate the activation trends of M1 and M2 types of microglia cells. Results No significant changes were found in the microglial cells in the spinal cord dorsal horn of rats in the sham-operation group during 14 days after operation. In the sciatic nerve ligation group at 1 day after operation, no significant change was observed in the number of microglial cells, but the expression of marker of M1 microglia was significantly increased. At 7 and 14 days after operation, the number of microglial cells and the expression of M1 microglia marker in the spinal cord dorsal horn were increased significantly. Conclusions Microglia activation in the spinal dorsal horn starts at the first day after sciatic nerve injury, and lasts at least for two weeks after the operation. M1 microglia activation dominates during this period.