Objective: To explore the changes and correlation of brain fractional amplitude of low frequency fluctuation (fALFF) and cognitive function in children with first-episode unmedicated attention deficit and hyperactivity disorder (ADHD). Methods: The case data of 46 children with first-episode unmedicated ADHD were retrospectively analyzed and included in the observation group, and 42 healthy children undergoing physical examination during the same period were selected as the control group. The differences in cognitive function (completed category count, persistent response count, persistent error count, and error response count of Wisconsin Card Sorting Test (WCST), and error rate of interference response inhibition evaluated by Simon task) and fALFF in brain regions were compared between the two groups. Pearson correlation analysis was used to analyze the correlation between cognitive function and fALFF in abnormal brain regions in children with ADHD. Results: Compared with the control group, the observation group had a lower completed category count, and higher persistent response count, persistent error count, error response count and interference response inhibition error rate (P<0.05). Compared with the control group, the fALFF values in the left superior postcentral gyrus and the right medial superior temporal gyrus were decreased in the observation group, while the fALFF values in the left caudate nucleus, posterior cingulate gyrus and right superior temporal gyrus were increased (P<0.05). Pearson correlation analysis showed that fALFF values in the left superior postcentral gyrus and right medial superior temporal gyrus of ADHD children were negatively correlated with error rate of interference response inhibition (P<0.05). Conclusion: There is a certain correlation between cognitive function changes and brain fALFF in children with first-episode unmedicated ADHD, and cognitive impairment may be related to abnormal changes in brain regions.