Abstract: Accurate assessment of the operational risk of AC-DC hybrid transmission systems can not only discriminate the transmission capacity and stability of AC-DC channels,but also provide an effective basis for the stability analysis and operation control of the transmitting and receiving systems. This paper proposes a "fuzzy cloud" risk assessment method based on the reliability series-parallel model of the key components of an AC-DC hybrid transmission system. First,the serial-parallel topology model of operation risk analysis strategy for AC and DC transmission system is studied, and a model of operation risk evaluation index system for AC-DC hybrid transmission system is established. Then,the interaction of the operation risks of each AC/DC hybrid transmission subsystem is studied. The fuzzy theory is used to determine the importance of the subsystem in the overall operation risk to realize the identification of key influencing factors. Next,considering the uncertain characteristics of renewable energy transmission in AC and DC transmission systems,the cloud model is used to solve each comprehensive determination of the risk level of the indicator so as to judge the overall risk level of the transmission system,by which a real-time quantitative assessment of the risk level of the AC/DC transmission system is achieved. Finally,based on the actual operating data of an AC/DC hybrid transmission system in Northeast China,an AC/DC hybrid transmission is system operation risk evaluation model established. The simulation results show that the fuzzy cloud risk evaluation method proposed in this paper can provide effective data support for the security risk assessment,operation stability control,and transmission capacity optimization of AC-DC hybrid transmission systems.