Objective To explore the clinical application of functional near-infrared spectroscopy （fNIRS） in distinguishing patients with generalized anxiety disorder（ GAD） alone from those GAD comorbid with attention deficit hyperactivity disorder（ ADHD）. Methods From October 2023 to May 2024， 34 adult patients with GAD alone were recruited as a control group， and 35 adult patients with GAD comorbid with ADHD were recruited as a study group from the outpatient and inpatient departments of the Second People's Hospital of Guizhou Province. The Adult ADHD Self-Report Scale（ ASRS） was used to assess participants' attention deficit and hyperactivity/impulsivity symptoms， while the Generalized Anxiety Disorder-7（ GAD 7） was employed to evaluate anxiety symptoms. FNIRS based on the 1-BACK task was used to detect changes in oxy-hemoglobin（ Oxy-Hb） concentration in the prefrontal cortex（ PFC） of both groups. Results The scores of the ASRS attention deficit subscale， hyperactivity subscale， and GAD-7 in study group were higher than those in control group， and the differences were statistically significant（ all P＜0.05）. Behavioral data indicated that there were no statistically significant differences in reaction time and accuracy between the two groups during the 1-BACK task（ all P＞0.05）. During the 1-BACK task， compared with control group， patients in study group exhibited statistically enhanced activation in multiple regions of PFC ［false discovery rate（FDR）- corrected P ＜ 0.05］， including the triangular inferior frontal gyrus（ channels 7，8，9）， middle frontal gyrus（ channels 15， 43）， dorsolateral superior frontal gyrus（ channels 16， 17）， and precentral gyrus（ channel 11）. However， when conducting a rank ANCOVA analysis with GAD-7 scores as a covariate to control for anxiety severity， all previously observed between-group channel differences were found to be statistically insignificant（ all P ＞ 0.05）. Additionally， the effect of GAD-7 as a covariate did not reach statistical significance across all channels （all FDR-corrected P ＞ 0.05）. Conclusions The severity of anxiety may serve as a key mediating variable influencing prefrontal cortical activation. When utilizing fNIRS to distinguish between GAD alone and GAD comorbid with ADHD， careful control of this confounding factor is essential.