Compared with a line commuted converter HVDC (LCC-HVDC), a hybrid cascade DC system at the sending end has advantages such as high flexibility and strong adaptability to weak networks. However, the mechanism of transient voltage fluctuations in the AC system after commutation failure is still unclear. To this end, a quantitative relationship between transient voltage and system strength, as well as power imbalance, is established. The influence of active and reactive power imbalance on transient voltage is considered, and the expression for transient voltage change rate is derived. The transient voltage characteristics of the commutation bus during commutation failure and recovery are clarified. Secondly, in order to suppress transient overvoltage of the converter bus, the expressions of LCC DC current command value and modular multilevel converter (MMC) reactive power command value are derived, and a collaborative suppression strategy based on multiple inverters is proposed. Finally, a simulation model is built based on the PSCAD/EMTDC simulation platform to verify the correctness of the mechanism analysis and the effectiveness of the suppression strategy.