Objectives To explore the correlation between plasma amyloid β protein 40， 42（ Aβ40， Aβ42） levels and their ratios in depressed patients without cognitive impairment. Methods A total of 70 depressed patients without cognitive impairment who had not gone through systemic treatment were selected as the experimental group. During the same period， a total of 70 healthy cases were selected as the control group. The plasma levels of Aβ40 and Aβ42 in each group were measured. The Hamilton Depression Scale 24（ HAMD-24） was used to assess the severity of depression. Results There was no significant difference in plasma Aβ40 level between patients and controls（ P=0.39）. The level of plasma Aβ42 in the patient group was significantly lower than that in the control group（ P=0.02）， and the level of Aβ40/Aβ42 was significantly higher than that in the control group（ P=0.01）. The results were not affected by age and the differences were statistically significant. Aβ42 level was negatively correlated with HAMD24 total score（ r=-0.401， P=0.015）， while Aβ40/Aβ42 was positively correlated with HAMD-24 total score（ r=0.461， P=0.005）. Conclusions Depressed patients without cognitive impairment have decreased serum Aβ42 levels and increased Aβ40/Aβ42 levels. The changes of Aβ42 and Aβ40/Aβ42 levels may be associated with the onset of depression.