Objective: The study aims to explore the neural mechanism of cognitive differences in college students with posttraumatic stress disorder under verbal fluency task based on functional near infrared spectroscopy (fNIRS), so as to provide neuroimaging support for the evaluation, diagnosis and treatment of posttraumatic stress disorder(PTSD). Methods: Posttaumatic Stress Disorder Checklist Combat(PCL-C) was used to screen the subjects, including 21 students in PTSD group (PCL-C≥38) and 30 students in control group from September to Novenber in 2020. A 53 channel near infrared spectroscopy device was used to collect cerebral blood oxygen signals under the verbal fluency task, and correlation analysis, Mann Whitney U test and independent sample t test were performed on the results. Results: The difference in the total average score of PCL-C Scale between PTSD group and the control group(46.38±6.96，25.57±6.09) was statistically significant ( t=11.33, P <0.05). Correlation analysis showed that Avg-HbO in left dorsolateral prefrontal lobe was negatively correlated with PCL-C Score ( r=-0.37, P <0.05). Mann Whitney U test showed that in the left dorsolateral prefrontal lobe (Ch6), the Avg-HbO change in PTSD group [0.19(-0.09, 0.86)mmol/(L〖KG*7〗·mm)] was significantly lower than the control group [0.79( 0.37 , 1.47)mmol/(L ·mm)] ( Z=2.16, P <0.05), which was statistically significant. Conclusions The degree of PTSD was negatively correlated with the index of oxygenated hemoglobin in the left dorsolateral prefrontal lobe, and the oxygenated hemoglobin content in the PTSD group was lower than that in the normal group. In the future, fNIRS may be used to collect blood oxygen signals from the left dorsolateral prefrontal lobe in cognitive tasks to provide imaging evidence for the identification of PTSD.