Objective To explore the difference of plasma endogenous opioid peptide level in patients with depression and its relationship with the severity of depression or anxiety, and analyze whether endogenous opioid peptide is helpful to the diagnosis and treatment of depression. Methods A total of 96 patients with depression who visited our hospital from June 2020 to February 2023 were enrolled as the depression group, and 50 healthy subjects were recruited as the control group. General demographic and clinical data [Hamilton depression scale 17(HAMD-17), Hamilton anxiety scale 14(HAMA-14), inventory of depressive symptomatology 30(IDS-30)] were collected. Peripheral blood was collected, plasma was extracted, and plasma β-endorphin (β-EP) and enkephalin (ENK) were compared between the two groups using enzyme-linked immunosorbent assay (ELISA). Due to the repeated impact of the epidemic, only 28 patients agreed to follow-up studies and blood collection plans. During the follow-up period, the treatment was in accordance with the plan made by outpatient physician, and there was not any research intervention. After 8 weeks of follow-up, 24 patients finished the studies, and their clinical data and blood samples were collected and compared. Results The plasma β-EP level in depression group was significantly higher than that in control group [49.48（36.79,61.14） ng/mL vs 30.84（23.51,38.14）ng/mL， P<0.05], while the plasma ENK level was significantly lower than that in control group [15.85（10.51,19.56）ng/mL vs 23.72（21.44,26.98）ng/mL， P<0.05]. Correlation analysis showed that there was no significant correlation between β-EP and ENK levels at baseline and the scores of the three scales (P > 0.05). There were no significant differences in plasma β-EP and ENK levels between cured group and non-cured group after 8-week of treatment compared to baseline (P > 0.05). Conclusions β-EP and ENK are expected to be biomarkers of depression.