Abstract: In order to analyze the cascading failure risk of flexible interconnected power distribution systems caused by hidden fault protection of flexible multi-state switch（FMS）, this paper first introduces its physical structure. Then, the steady-state characteristic, the transient characteristic of AC feeder failure, and FMS control and protection strategies are analyzed. The uncertain factors that cause the protection malfunctions are proposed and modeled. On this basis, the mechanism of cascading failure formation is analyzed, and the cascading failure risk indicators of flexible interconnected power distribution networks are established. The probabilistic indicators based on Monte Carlo method and the calculation methods based on optimal load shedding failure consequences are also proposed. Taking the modified IEEE 33-bus system as an example, the risk of cascading failures in the system is evaluated, and the influence of the failure location and the penetration rate of distributed generators on the risk is analyzed.