Objective To explore the relationship between event-related potentials P300 and the severity of depression. Methods A control study was conducted among 28 moderate depression patients， 41 severe depression patients and 70 healthy controls. Results The latency of P3 was positively correlated with the scores of Hamilton Depression Scale（ HAMD-17）（ r=0.514，P＜0.01）. Compared with the healthy control group， the moderate group and severe group showed prolonged latencies of N2 and P3， and decreased P3 amplitude， with statistical significance（ P ＜ 0.05）. Compared with moderate group， the severe group showed prolonged P3 latency（ P=0.017）. Conclusions The cognitive impairment might be associated with the severity of the depression， and the P3 latency might be used as an indicator of severity in depression.