Background Surgical site infections (SSIs) after spinal surgery are a major diagnostic and therapeutic challenge for spinal surgeons. Polymicrobial infections after spinal surgery seem to result in poorer outcomes than those caused by one species, due to complementary resistance to antibiotics. However, data are limited regarding the clinical manifestations and severity comparison between polymicrobial and monomicrobial infections. The aim of this study was to investigate the difference of clinical manifestations and severity between polymicrobial infections and monomicrobial infections after spinal surgery. Methods 67 surgical site infected patients after spinal surgery were studied, 20 patients with polymicrobial infections and 47 patients with monomicrobial infections. Pathogenic bacteria identified were counted and classified. Age, sex and body mass index (BMI) were compared between the two groups to identify homogeneity. The two groups were compared for clinical manifestations by postoperative time to infection, infection site, incisional drainage, incisional swelling, incisional pain, neurological signs, temperature, white blood cell count and percentage of neutrophils. Finally, the two groups were compared for severity by hospital stay, number of rehospitalizations and number of debridements. Results Polymicrobial infections made up 29.9 percent of the surgical site infections after spinal surgery, and most polymicrobial infections (70.0%) were caused by two species of bacteria only. There was no difference between the 2 groups in clinical manifestations and severity. A total of 96 bacterial strains were isolated from the spinal wounds. 60/96 strains were gram-positive and 36/96 strains were gram-negative pathogenic bacteria. Staphylococcus aureus, Staphylococcus epidermidis, Escherichia coli and Enterobacter cloacae were cultured in order of the frequency. Conclusions Most polymicrobial infections were caused by two bacterial species. There was no difference in clinical manifestations or severity between polymicrobial infections and monomicrobial infections after spinal surgery.