Abstract: The solution of automatic ground convert technology can shorten the power failure time and reduce the locomotive speed loss, which is in line with the development trend of China′s electrified railway. This paper takes the automatic ground convert equipment as the research object, and deduces the mathematical expressions of thyristor closing overvoltage and cut-off overvoltage by establishing the equivalent model. Then it analyses the transient overvoltage mechanism and puts forward the suppression technology scheme accordingly. Based on MATLAB/Simulink simulation platform, an integrated simulation model of electric locomotive and automatic ground convert device is constructed, and the correctness and validity of the over-voltage transient process of the locomotive and its suppression technology scheme are verified.