Objective: To examine the clinical manifestations, imaging features, potential pathological mechanisms, differential diagnosis, and clinical significance of atypical Alzheimer's disease (AD), with the objective of enhancing comprehension of atypical AD and elevating the standard of clinical diagnosis. Methods: The clinical manifestations, laboratory findings and imaging features, and differential diagnosis of three patients with atypical AD were reviewed, and their pathological mechanisms were analyzed accordingly. Results: There was a decrease in Aβ and an increase in Tau in all three patients' cerebrospinal fluid (CSF) tests. The patient with frontal variant of AD (FvAD) started with memory loss and mental behavior abnormalities, and the brain Magnetic Resonance Imaging (MRI) showed symmetrical atrophy of bilateral frontal lobe, temporal lobe, parietal cortex and hippocampus. The patient with logopenic variant primary progressive aphasia (LvPPA) has the onset of speech clumsiness. Brain MRI shows that the left lateral fissure is wider than the right, and bilateral hippocampal atrophy is relatively mild. The patient with posterior cortical atrophy (PCA) presented with memory loss and visuospatial impairment. Brain MRI showed bilateral partooccipital lobe atrophy, widening of the right lateral fissure, and relatively mild bilateral hippocampal atrophy. Conclusions: Atypical AD are starting with speech impairment, mental behavior abnormalities, and visuospatial disorders. Because the dysmnesia symptoms of atypical AD are not typical, it is not easy to identify them early, and it is easy to be confused with frontotemporal dementia, cortical basal ganglia degeneration and Lewy body dementia. The clinical diagnosis requires a comprehensive consideration of detailed medical history and neurological examination, combined with neuropsychiatric scales, imaging, and biomarkers.