The mechanism of over-voltage is analyzed according to the problems that insulated equipment is breakdown and threat to traffic safety when locomotive goes through overlapping electrical sectioning device which generate over-voltage. The model of overlapping electrical sectioning device is established by using PSCAD software. By comparing the size of overvoltage when locomotive goes through seven and eight-span overlapping electrical sectioning device, and RC protection devices is installed in seven-span overlapping electrical sectioning device .It can be introduced : inhibitory effect is significant by installing RC protection devices in the neutral line for over-voltage . Inhibitory effect will be more obvious by installing RC protection devices in the neutral line for over-voltage compared to seven-span overlapping electrical sectioning device.