To explore the characteristics of event-related potential P300 in patients with depression and its correlation with the Patient Health Questionnaire (PHQ-9) scores. Methods: Thirty-six patients with depressive disorder were included into the depression group, and 34 healthy subjects were recruited as the control group. PHQ-9 scores were obtained from the subjects, and event-related potential P300 was collected under the visual oddball task in both groups. The difference of latency and amplitude of P300 between the two groups and its correlation with PHQ-9 scores was analyzed. Results: The P300 amplitude of the depression group compared with that of the normal control group was significantly reduced in the C4, P4, T4, and T6 electrodes (P=0.020, 0.015, 0.033, 0.013). PHQ-9 was negatively correlated with the amplitude of P300 at the C4, P4, F8, T4, and T6 electrodes (r=－0.267, －0.252, －0.285, －0.287, －0.335, P=0.026, 0.036, 0.017, 0.016, 0.005). Conclusion: The change of event-related potential P300 is a possible objective indicator to reflect the cognitive impairment of patients with depression. Based on the severity assessment by PHQ-9, it may have some guiding significance in indicating cognitive impairment in patients with depression.