Voltage interaction between the rectifier and inverter buses is recognized as a critical factor in embedded direct current (EDC) transmission systems, where at least two ends are within a single synchronous AC network, as it significantly affects power flow distribution, voltage stability, and power system planning. Conventional methods for evaluating voltage interaction are insufficient to accurately represent the complicated interplay between responses of the AC-DC network and the internal controllers within EDC transmission systems. To address this issue, this paper proposes an analytical calculation method of a novel voltage interaction evaluation index for various types of EDC transmission systems, which enables precise evaluation of the voltage interaction between the rectifier bus and inverter bus in an EDC transmission system. The proposed method comprehensively accounts for the influence of voltage interaction under small disturbances through the AC network, as well as the influence of voltage interaction under the same disturbance between converter buses through internal controller responses. Numerical simulations are used to analyze the parametric dependence of the index, and its accuracy is demonstrated through dynamic simulation.