Objective To explore the influencing factors of cognitive impairment in patients with obstructive sleep apnea hypopnea syndrome (OSAHS). Methods Enrolled in the study were 175 OSAHS patients (96 severe and 79 mild to moderate) diagnosed by polysomnography in the Sleep Center of Respiratory Department of Nanjing First Hospital from January 2018 to December 2019. According to the Epworth Sleepiness Scale (ESS) score and sleepiness symptoms, the patients were divided into OSAHS patients with excessive daytime sleepiness (EDS) and patients without EDS. The general information was collected, and the Montreal Cognitive Assessment (MoCA) was used to evaluate the cognitive impairment of the patients. OSAHS patients with and without EDS were compared in the cognitive impairment scores, and the related factors were analyzed. SPSS statistics 22.0 was used for statistical analysis. Comparison between the two groups was performed using t test or χ²test depending on data type. Correlation analysis was performed using Pearson test. Multivariate regression analysis was used to evaluate the factors related to MoCA and ESS.Results Oxygen desaturation index (ODI), lowest nocturnal oxygen saturation (LSaO₂) and percentage of total sleep time with oxygen saturation <90% (TS90%) were significantly different between severe OSAHS patients with and without EDS (P＜0.05). Oxygen desaturation index (ODI) and TS90% in mild to moderate OSAHS patient with EDS were higher than those without EDS (P＜0.05). Visual spatial executive ability, attention, delayed memory, abstract ability and orientation ability in the severe OSAHS patients were lower than mild to moderate OSAHS patients(P＜0.05). In severe OSAHS patients, MoCA in patients with EDS was lower than that in patients without EDS, and there was significant differences in attention and delayed memory(P＜0.05). MoCA in mild to moderate OSAHS patients with EDS was lower than that of patients without EDS, and difference with statistical significance was only seen with attention(P＜0.05). MoCA was found to be negatively correlated to ESS, AHI, ODI and TS90% but positively to LSaO₂, the differences being statistically significant (r=-0.432, -0.511, -0.539,-0.420、0.458) (P＜0.01 for all). ESS was positively correlated with body mass index, abdominal circumference, waist-to-hip ratio, ODI and TS90%, and negatively with LSaO₂, the differences being statistically significant (r=0.262,0.299,0.460,0.538,0.498, -0.417) (P < 0.05 for all). Multivariate linear regression analysis showed that MoCA was independently correlated with ESS and ODI (R²=0.325,P＜0.05). Conclusion EDS is closely related to cognitive impairment in OSAHS patients. MoCA is lower in severe OSAHS patients or mild to moderate OSAHS patients with EDS than that of patients without EDS, and attention deficit is the most obvious. Therefore, early identification of cognitive impairment early intervention such as CPAP in the OSAHS patients can avoid further deterioration of cognitive function.