For Guangdong back-to-back volatage source converter - high voltage direct current (VSC-HVDC) project, there are only two outgoing lines on the AC side and only one lower level substation, the risk of the last circuit breaker appearing in the converter is high. In the project, the last circuit breaker protection will be configured to prevent equipment overvoltage damage. In order to reduce the overvoltage characteristics and improve the action time margin of the last breaker protection, based on the topology and control strategy of Guangdong back-to-back VSC-HVDC project, the overvoltage mechanism and characteristics under three-phase and single-phase tripping conditions are studied. Then the range of amplitude limit of the positive and negative sequence modulation signal is analyzed, and a method to reduce overvoltage characteristics by optimizing the amplitude limits is proposed, and EMTDC simulation is carried out. The results show that the overvoltage characteristics of back-to-back VSC-HVDC are strongly related to the positive and negative sequence control, and the overvoltage of single-phase tripping of the last breaker is more serious than that of three-phase tripping, the saturation characteristic of transformer will restrain overvoltage, but it will make the control response more complex. The method proposed in this paper can significantly improve the action time margin of the last breaker protection action.