External steep-front transient overvoltages may introduce high-frequency electromagnetic disturbances into modular multilevel converter-based high-voltage direct current (MMC-HVDC) systems, resulting in additional transient voltage stresses on converter valves and their sub-modules. This paper investigates the propagation, coupling, and transient voltage distribution characteristics of external high-frequency disturbances in MMC converter valves using a multi-scale wideband equivalent model covering the “sub-module–converter valve–converter station” hierarchy. A wideband equivalent model of a 4.5 kV/3 kA press-pack insulated gate bipolar transistor (IGBT)-based sub-module is developed and integrated with the distributed parasitic parameters of the valve tower and the high-frequency characteristics of converter-station components. To investigate the converter-valve response under different transient conditions, a representative lightning impulse is considered as an engineering transient condition, while a controlled fast-front impulse is employed to investigate the intrinsic high-frequency propagation and resonance characteristics of the distributed converter-valve network. Simulation results demonstrate that external transient disturbances can propagate into MMC converter valves through grounding parasitic capacitances and distributed coupling paths, resulting in additional differential-mode transient voltage stresses at sub-module terminals. Sub-modules closer to the disturbance source experience higher transient voltage peaks and larger dv/dt values. Moreover, multiple local resonance bands are identified within the valve tower, with high-frequency oscillatory components above 10 MHz being strongly influenced by the distributed parasitic network. Parameter analysis further indicates that the sub-module stray inductance has an important influence on the magnitude and oscillatory characteristics of the induced transient voltage. The proposed multi-scale modeling approach provides a practical method for evaluating high-frequency transient voltage stresses in MMC-HVDC converter valves and provides theoretical support for insulation coordination, converter-valve structural optimization, and transient reliability design of HVDC transmission systems.