Abstract: The research on commutation failure suppression strategy of UHVDC system converter has important engineering significance. Based on the coupling characteristics of the high and low side of the inverter side in the UHVDC system under hierarchical connection,this paper analyzes the mechanism of commutation failure caused by the increase of the overlap area,and proposes a coordinated control strategy to suppress the commutation failure of the non-fault layer converter in the UHVDC system under hierarchical connection. The strategy compensates the overlap area required by the non-fault layer converter after AC fault,so that the thyristor has enough extinction area to complete the de dissociation process,so as to prevent commutation failure. Finally,an actual system model is built based on PSCAD/EMTDC,and the simulation results show that the proposed coordinated control strategy can effectively suppress the commutation failure of the non-fault layer converter under symmetrical and asymmetric faults of the receiving end grid under different fault degrees,and improve the stability of the system.