Objective To study the characteristics of cognitive potential in patients with depressive episode of bipolar disorder by event-related potential P300. Methods Totals of 32 patients with depressive episode of bipolar disorder （patient group， including 8 patients with bipolar I disorder and 24 with bipolar II disorder） and 30 healthy controls （control group） were recruited. P300 test was conducted using the classic visual oddball paradigm. Behavioral data and P300 components were compared and analyzed. Results The accuracy of keystroke in the patient group ［（85.42±9.33）%］was lower than that in the control group ［（89.88±6.84）%］， and the difference was statistically significant （P＜ 0.05）. The amplitude of multiple electrode sites in the patient group was lower than that in the control group， and the difference was statistically significant （P＜0.05）. Further analysis showed that the P300 latency of bipolar depressive episode patients with psychotic symptoms at the electrode site FP1 ［（570.00±208.65） ms］， F7 ［（505.20±184.50） ms］was longer than that of patients without psychotic symptoms ［（411.59±196.52）， （371.76±142.80） ms］， and the difference was statistically significant （P ＜ 0.05）. Conclusions The cognitive function of patients with bipolar depressive episode is impaired to a certain extent， which is mainly manifested in the decrease of amplitude of visual P300， the latency at sites FP1 and F7 are longer in the bipolar depressive episode patients with psychotic symptoms than in the group without psychotic symptoms.