Abstract: Based on ±1100kV Changji-Guquan UHVDC hierarchical con-nection mode transmission project, the simulation model of UHVDC considering interlayer coupling is set up by PSCAD/EMTDC electromagnetic transient simulation software. The difference of the AC system strength between the hierar-chical connection mode and the traditional connection mode is compared, and the variation characteristics of the effective inter-layer short circuit ratio of the hierarchical connected AC system under different coupling strengths are simulated. By studying the influence law of the interlayer impedance on the steady-state system operation characteristics and the system commutation failure characteristics under the fault, the interaction between the different AC systems of the inverter side under steady state and transient state is analyzed. The results of this paper can provide theoretical support for the operation stability of UHVDC hierar-chical connection mode transmission system.