In order to clarify the impact of changes in the longitudinal slope of undersea tunnel on the driver's electroencephalography (EEG) signal, real vehicle experiments are carried out, using EEG instrument, gradient recorder, video recorder and other experimental equipments to collect data, such as driver's EEG and the data on the longitudinal slope of the undersea tunnel. The EEG power spectrum and EEG power are selected as parameters, and the EEG signal change law is analyzed when the driver passes through the undersea tunnel's slope change area; the slope change areas are grouped according to the difference of the up and down slopes, and the mathematical models of the longitudinal slope change and the change of EEG β wave power have been established respectively. The study results show that the brain activity level of drivers increases significantly when passing through the gradient zone; the frontal area is the brain area which is most sensitive to the changes in longitudinal slope; the longitudinal slope and its change are positively correlated with the change of the driver's EEG β wave power. The smaller the longitudinal slope, the smaller the EEG β wave power change; the lower the driver's psychological pressure, the better the driving safety.