During the operation of group trains,the trains frequently pass through the neutral zone. Then,the generated closing overvoltage will cause serious insulation damage to the traction power supply system and the trains' high-voltage equipment box. In order to qualitatively analyze the closing overvoltage characteristics of group trains,the equivalent simplified dynamic circuit of the traction power supply system of group trains is established according to the principle of circuit equivalent simplification. The expression of the train terminal voltage at the moment of closing is solved by reasonable assumptions,which reveals the formation reasons of the closing overvoltage in terms of the qualitative relationship. Based on the transient model of the group train traction power supply system,the influence of different train numbers,distribution locations and operating power on the closing overvoltage of group trains is investigated. Finally,a simulation model is built in PSCAD/EMTDC to verify the correctness of the theoretical analysis results. The results show that with the increase of the number of group trains,the centralization of the operation location,and the traction power,the peak closing overvoltage decreases. The results can also effectively guide the closing overvoltage mitigation of group train neutral zone,which reduces the closing overvoltage when the group trains pass through the neutral zone.