Objective: To explore the clinical characteristics of early on-set Alzheimer's disease (AD), improving the understanding of the diagnosis and treatment of the disease. Methods: This was a case-retrospective study on the data of clinical manifestations, neuropsychology, dynamic EEG and structural MRI of 18 early on-set AD patients who were proved with cerebral spinal fluid AD biomarkers. Results: The average age of 18 patients(6 males,12 females) with AD was 55.4 years old, and the average course of the disease was 5.6 years.17 patients suffered memory loss as initiate symptom, and 9 patients had family histories of dementia. The average score of MMSE was 14.3, and there existed 12 patients with moderate and severe dementia. Among 13 patients who accomplished gene whole-exon sequencing, we found 3 cases with PSEN1 mutations, 4 cases with SORL1 mutations, 1 case with ABCD7. At the same time, the carrying frequency of APOE gene ε2, ε3, ε4 in 18 patients was 1/36, 27/36 and 9/36, respectively. All 13 cases who accepted dynamic EEG testing showed abnormal EEGs with epileptiform discharge. The hippocampus atrophy of 8 patients was not significant. Conclusion: The clinical characteristics of the early on-set AD were hidden onset but developed fast. At the time of treatment, it has often been developed into severe dementia, with decline memory as initiate symptom and most patients have family history of dementia. Epilepsy discharge of EGG were detected more commonly in these patients, whose carrying rates of AD causing genes and AD risk genes were also higher than normal ones. And some cases had less hippocampus atrophy.