Objective: To explore the utility of ¹⁸F-FDG and ¹⁸F-AV45 PET/CT in patients with varying cognitive impairment severities. Methods: This retrospective study included 202 patients with cognitive impairment (70 males, 132 females; aged 41-91 years) who underwent both ¹⁸F-FDG and ¹⁸F-AV45 PET/CT between December 2020 and March 2025. Patients were classified into mild cognitive impairment (MCI, n=81), mild Alzheimer’s disease (AD, n=75), and moderate-to-severe AD (n=46) groups. A healthy control (HC) group (n=23) was also included. Regional ¹⁸F-FDG Z-scores and ¹⁸F-AV45 SUVR values were processed using Cortex ID software. Statistical analyses included Cohen's Kappa, ANOVA, Kruskal–Wallis test, Bonferroni correction, Pearson correlation, and ROC analysis. Results: Visual analysis method of PET imaging showed that the sensitivity in diagnosing MCI was 74.07% (Kappa=0.34, P<0.001); the sensitivity for diagnosing AD was 96.69% (Kappa=0.78, P<0.001). Semi-quantitative analysis revealed: With the progression of the patients' condition, the ¹⁸F-FDG Z-score of the whole brain and each brain region gradually decreased (H values: 25.53-85.31, all P<0.001). The ¹⁸F-AV45 SUVR gradually increased (H values: 37.93-49.08, all P<0.001). There was no statistically significant difference in ¹⁸F-AV45 SUVR among the AD subgroups (all P>0.05). ROC analysis identified the posterior cingulate gyrus as the most discriminative region for ¹⁸F-FDG in differentiating MCI from HC and AD from HC (AUC=0.68-0.99) and the parietal lobe for mild AD vs. MCI (AUC=0.71). For ¹⁸F-AV45, the posterior cingulate–precuneus region best distinguished MCI/AD from HC (AUC=0.87-0.99), and the occipital lobe best differentiated moderate-to-severe AD from MCI(AUC=0.66). Conclusions: ¹⁸F-FDG and ¹⁸F-AV45 PET/CT imaging can non-invasively evaluate brain Aβ deposition and neurodegenerative changes; Both their visual and semi-quantitative analysis methods can provide important imaging evidence for the early diagnosis and pathophysiological evolution of MCI and AD; ¹⁸F-FDG PET/CT is superior in evaluating disease severity and cognitive function.