Complexity of the railway operation environment makes it possible to break the contact line, and the broken contact wire falling on the surface of the motor train unit will lead to the overvoltage of the cab body. In order to evaluate the over-voltage damage of Carbon Fiber Reinforced Polymer/Plastic (CFRP) cab body due to its poorer conductivity than that of traditional aluminum alloy cab body, the circuit model of CFRP train body is established based on the semi-CFRP semi-aluminum alloy structure design. The transmission characteristics of surge overvoltage in different positions of the cab body are solved by simulation. The maximum voltage reaches 26 kV, and attenuates rapidly in 60 μs, which is a potential safety hazard for on-board equipment. It is proposed that the grounding wires of on-board equipment should be connected directly near the grounding brush of cab body, which provides a theoretical basis for the design of overvoltage protection of CFRP cab body.