Objective To explore the effect and injury mechanism of reactive oxygen species (ROS) after spinal cord injury (SCI) through detecting the dynamic changes of malonyldialdehyed （MDA）content in spinal cord and observing neurocyte apoptosis and correlation apoptosis factor expression after SCI. Methods Totally 132 adult SD male rats were randomly divided into three groups: sham group, SCI group, methylprednisolone (MPSS) group. The SCI of SD rats was performed by Allen’s weight dropping way to impact on the posteriors of spinal cord T10. The contents of MDA were determined by chromatometry, the expression of Caspase-3 and Bcl-2 family in the injured spinal cord was detected by immunohistochemical staining; Apoptotic cells were detected by using fluorometric terminal deoxynucleotidyl transferase mediated dUTP nick end labeling (fluorometric TUNEL) staining. Results The content of MDA in the injured cord increased significantly after SCI, reached the peak at 6 hours and 3 days post-injury, then dropped down gradually, then was back to the normal level after 7 days. The number of TUNEL labeling positive cells of SCI group increased at 6 hours post-injury, reached the peak at 3 days, then dropped down gradually; Bcl-2, Bax protein began to increase at 6 hours post-injury, reached the peak at 5 days after injury, then dropped down gradually. Caspase-3 protein began to increase at 6 hours post-injury, reached the peak at 3 days after injury, then dropped down gradually. The content of MDA, the number of TUNEL labeling positive cells, the expression of Caspase-3 and Bax of MPSS group decreased significantly than that of SCI group at the same time, respectively, while Bcl-2 protein was up-regulated after administration of MPSS.Conclusion ROS could promote the expression of Caspase-3 and degrade the ratio of Bcl-2/Bax to induce apoptosis of neurocyte, which might play significantly role in the process of secondary SCI. In addition, MPSS exerts neuroprotective effects against ROS toxicity, which might be of importance and might contribute to their clinical efficacy for the treat