To investigate the alertness and task processing speed impairment status in young-mild aged men with obstructive sleep apnea hypopnea syndrome (OSAHS), and analyze its influencing factors. This prospective study recruited 251 snoring patients aged 18 to 59 (38.9±7.6) years in the Sleep Center of the Second Affiliated Hospital of Soochow University from July 2020 to September 2021 and all patients were diagnosed by polysomnography (PSG). Clinical information, Epworth Sleepiness Scale (ESS) and PSG date were collected. All patients were assessed with the Montreal Cognitive Assessment (MoCA) questionnaires, Mini-mental State Examination (MMSE) and Computerized Neurocognitive Assessment System which includes the reaction time of Motor Screening Task (MOT) for alertness, the reaction time of pattern recognition memory (PRM), spatial span (SSP) and spatial working memory (SWM) for task processing speed. Based on AHI tertiles, all patients were divided into Q1 group (AHI<15 times/h, =79), Q2 group (15 times/h≤AHI0.05). There were statistically significant among-group differences in the body mass index (BMI), ESS, prevalence of hypertension and complaints of daytime sleepiness (<0.05). Compared with Q1 and Q2 group, the arousal index (ArI), oxygen desaturation index (ODI),the proportion of non-rapid eye movement phase 1 and 2 (N1+N2) and percentage of total sleep time with oxygen saturation level<90% (TS90) of Q3 group were higher (all 0.05). Compared with the Q1 group, the task processing speed and alertness were worse in Q3 group, as shown by slower PRM immediate and delayed reaction time, SSP reaction time and MOT reaction time (all <0.05). The total time of SWM in Q2 group was slower than that in Q1 group (<0.05). Multiple linear stepwise regression showed that years of education (β=-40.182, 95%:-69.847–10.517), ODI (β=3.539, 95%: 0.600-6.478) were the risk factors of PRM immediate reaction time. Age(β=13.303,95%: 2.487-24.119), years of education(β=-32.329, 95%:-63.162–1.497), ODI (β=4.515, 95%: 1.623-7.407) were the risk factors of PRM delayed reaction time. ODI was the risk factor of SSP reaction time (β=1.258, 95%: 0.379-2.137). TS90 was the risk factor of MOT reaction time (β=1.796, 95%: 0.664-2.928). The early cognitive impairment in young-mild aged OSAHS patients was manifested in decreased alertness and task processing speed, and intermittent nocturnal hypoxia was its influencing factor in addition to age and years of education.