Abstract: Objective To explore the role of neutrophil-to-lymphocyte ratio (NLR), Platelet-to-lymphocyte ratio (PLR), and mean platelet volume (MPV) in the diagnosis and judgment of the severity of obstructive sleep apnea syndrome (OSAS). Methods A retrospective case-control study was conducted among the patients with OSAS. The sleep parameters were recorded by using a polysomnography (PSG). The patients were assigned into mild, moderate and serious groups according to the apnea hypopnea index (AHI). The peripheral blood neutrophil count (WBC), lymphocyte count (LC), platelet count (PLT), platelet distribution width (PDW), NLR, PLR, MPV and other related data were detected and compared among the four groups. The correlation analysis of NLR, PLR, MPV and AHI was carried out by Pearson test, and the value of each index to the diagnosis of OSAS patients was evaluated by ROC curve. Results There were statistical differences in BMI, TC, TG, LDL, nocturnal mean heart rate, NLR, PDW, MPV, minimum oxygen saturation and mean oxygen saturation among the four groups (P<0.05). There was a significant correlation between NLR, PDW, MPV and AHI, P<0.05, however, there was no significant correlation between PLR and AHI (P>0.05). ROC curves analysis showed that the area under the curve (AUC) score was 0.836 for NLR, the best cutoff for ROC was 1.685, sensitivity was 65.5%, specificity was 92.3%, positive predictive value was 96.5%, and negative predictive value was 45.3%. Conclusion NLR, PDW and MPV play an important role in the judgment of the severity of OSAS.