| Objective To explore the application value of event-related potential P300 in cognitive dysfunction after traumatic brain injury. Methods From January to September 2021, a total of 36 patients with traumatic brain injury who were conservatively treated in the Neurosurgery Department of the Third Hospital of Mianyang and met the diagnostic criteria were selected as the research group, and 36 normal control groups were recruited from the family members and nurses of other patients in the hospital during the same period. Oddball paradigm was used to measure the event-related potential P300. Montreal Cognitive Assessment (MoCA) and Mini-Mental State Examination (MMSE) were used to assess the cognitive function of the subjects. P300 latency and amplitude, MoCA and MMSE scores were compared between the two groups, and the detection rates of P300 latency, MoCA and MMSE on cognitive dysfunction in patients with traumatic brain injury were compared. Results MoCA and MMSE scores in the study group were lower than those in the control group [(18.08±4.23) vs. (27.36±1.18), (22.53±3.49) vs. (28.11±1.07), t=-12.510, -9.041, P<0.05]. The latency of P300 in the study group was higher than that in the control group [(406.08±26.57)ms vs. (367.08±22.18)ms, t=6.665, P<0.05], and the amplitude was lower than that in the control group [(7.76±0.88)μV vs. (9.87±0.97)μV, t=-9.469, P<0.05]. In study group, the positive rate of P300 latency and the rate of MoCA on cognitive dysfunction were higher than that in MMSE (χ2=5.675, 7.604, P<0.05). Conclusion P300 can be used as an objective clinical indicator for evaluating cognitive dysfunction in patients with traumatic brain injury.