Objective To study the relationship between the serum homocysteine ( Hcy ) levels in patients with first-episode depression and event related potentials ( ERP ) P300.Methods 117 first-episode depressive patients were selected as the case group,and 110 healthy volunteers were selected as the health control group,the de-termination of serum Hcy levels in the two groups were detected by enzyme linked immunosorbent assay,measured by the Shenzhen Brain Master brain evoked potential system to complete the visual P300.Results The serum Hcy levels of the case group was (14.0 ±9.8)μmol/L,which was higher than (10.5 ±7.5)μmol/L of the health control group (t=-3.087,P <0.05).The N1,N2 incubation period of the case group were (91.4 ±18.6) ms,(235.4 ± 30.5)ms,respectively,which were longer than (88.6 ±13.0)ms,(204.3 ±20.3)ms of the health control group(t=1.305,8.990,all P<0.01);The P3 incubation period of the case group were (359.5 ±30.0)ms,which was longer than (327.2 ±41.8)ms of the health control group(t=6.685,P<0.05).The target P3,non-target P2 amplitude of the case group were (4.1 ±2.0)μV,(4.2 ±1.9)μV,respectively,which were singnificantly lower than (6.8 ± 3.8)μV,(6.6 ±3.6) μV of the health control group( t=-6.694,-6.485,all P<0.01).The P2 incubation period of Hcy case group was (194.4 ±13.6) ms,which was longer than (163.1 ±21.8) ms of the non Hcy case group(t=8.486,P<0.01).The P3 incubation period of Hcy case group was (357.0 ±24.5)ms,which was longer than (337.1 ±45.5)ms of the non Hcy case group(t=2.645,P<0.05).The target P3,non-target P2 amplitude of the Hcy case group were (5.4 ±1.5)μV,(5.2 ±1.2) μV,which were significantly lower than (5.8 ±3.8)μV, (5.4 ±3.2)μV of the Hcy case group(t=-0.745,-0.208,all P<0.01).Conclusion The serum Hcy levels in patients with first-episode depression increased, and P300 latency, amplitude decreased, the cognitive function is related to Hcy metabolism disorder and depression damage.