Abstract: Objective To investigate the EEG characteristics and related physiological mechanisms in patients first onset of depression during mental rotation tasks,and provide reference for the treatment of depression. Methods The patients with depression in our hospital from January,2013 to June,2014 were assigned into the observation group,while the subjects for physical examination were selected as control group.The clinical data was reviewed.E-prime software was used to determine the error rate and response time.The changes of brain potentials were detected by Brain AMP MR32 portable event-related potential recording system.The differences between the subjects with different rotation angle error rate and response time and P500 latency at P3,PZ and P4 and amplitude were compared,and the correlation analysis on P500 latency and amplitude with HAMD score were conducted. Results The error rate for both erect image and mirror image under different rotation angle in the observation group was significantly higher than that in the control group,the difference was statistically significant(P<0.05).In the observation group,the error rate for both erect image and mirror image increased with the rotation angle degrees,and the reaction time at different rotation angle degrees was longer than that in the control group,the difference was statistically significant(P<0.05);P500 latency at P3,PZ and P4 in the observation group was significantly longer than that in the control group,but the amplitude was lower than that in the control group,the differences were statistically significant (P<0.05);In the observation group,HAMD scores was positive correlated with the latency at P3 and the amplitude at P3 and PZ(P<0.05). Conclusion After mental rotation,the cognitive function in patients with depression can appear a certain degree of damage,the incubation period of P500 should prolong and the amplitude decline.HAMD scores show positive correlation with P3 and PZ incubation period and P3 amplitude.