With the advancement of the “dual carbon” strategy, China is vigorously developing new energy sources such as wind and solar power. New energy stations located at the edge of the power grid often lack support from AC power sources and rely on MMC-HVDC systems for grid connection. However, the widespread application of power electronic converters has led to high-frequency resonance issues. First, to address the high-frequency resonance problem in new energy stations connected via MMC-HVDC, a mathematical model of the MMC islanded system is established, including the main circuit and control components, as well as electromagnetic voltage/current transformers and their secondary cables. Next, the secondary cables of the electromagnetic voltage/current transformers are modelled using π-type circuits, and their characteristics are analyzed. Then, a stability analysis of the MMC islanded system is conducted, considering the secondary cables of the electromagnetic voltage/current transformers. The mechanism of high-frequency resonance in the islanded system caused by the transmission characteristics of the secondary cables of the electromagnetic voltage/current transformers is clarified. Finally, simulation verification is carried out in PSCAD/EMTDC. Combined with field tests, the accuracy of the proposed high-frequency resonance mechanism in the islanded system caused by the secondary cables of the electromagnetic voltage/current transformers is proved.