[Objective] To better understand how the structural characteristics of urban high-voltage cable tunnel networks affect fire propagation, this study aimed to provide a scientific basis for fire prevention, control strategies, and tunnel design optimization. Specifically, fire propagation behavior under different structural configurations and ventilation systems in cable tunnels was investigated. Key fire dynamics parameters analyzed included temperature distribution, smoke propagation, flame spread speed, and toxic gas concentrations for both single-layer and multilayer cable arrangements. [Methods] The investigation relied on three-dimensional fire dynamics simulations using Fire Dynamics Simulator software to explore fire propagation in urban high-voltage cable tunnels. Various tunnel structural configurations were analyzed, including single-layer and multilayer cable arrangements, with and without ventilation systems. Critical fire parameters, such as heat release rate (HRR), ceiling temperatures above the fire source, smoke flow patterns, and toxic gas concentrations, were examined under different fire scenarios. Numerical modeling provided detailed insights into how fire dynamics interact with tunnel structural features, emphasizing the significance of cable layout and ventilation. The study also explored how ventilation affects smoke behavior, assessing its influence on fire spread, temperature, and gas emissions. [Results] The results revealed distinct differences in fire behavior depending on tunnel structure and ventilation. Multilayer cable configurations caused ceiling temperatures above the fire source to rise significantly faster and reach higher peaks within 400 s compared to single-layer arrangements. This rapid increase in temperature indicated that the denser cable arrangement boosted the HRR, resulting in greater thermal effects. Smoke propagation was highly dynamic. During the early stages, it initially spread rapidly to one side of the fire source. Ventilation systems, however, altered this behavior by reversing the smoke flow direction over time. This reversal created localized temperature increases and more complex smoke distribution patterns. It also introduced cooler air into the system, influencing flame propagation and heat transfer dynamics. Toxic gas analysis showed that carbon monoxide levels were significantly greater for multilayer cables between 300 s and 500 s, indicating more incomplete combustion and increased hazardous gas emissions in these configurations. Flame propagation was faster, and heat transfer effects were more pronounced in multilayer configurations, highlighting the critical role of structural design in fire dynamics. These results underscore the heightened fire hazards posed by multilayer cable arrangements, including faster flame spread, greater thermal effects, and elevated toxic gas concentrations. [Conclusions] This study emphasizes the critical need for optimizing tunnel designs and ventilation systems to mitigate fire risk effectively in urban high-voltage cable tunnels. Multilayer cable arrangements notably increased heat release rates, toxic gas emissions, and flame propagation intensity, exacerbating fire hazards. While ventilation systems can positively influence smoke propagation and control localized temperatures, improper ventilation strategies may introduce risks, such as smoke flow reversal and uneven heat distribution. These findings provide essential technical insights for developing fire safety measures, highlighting the need for prevention and control strategies tailored to the structural and operational characteristics of urban high-voltage cable tunnels. By addressing these challenges, this research helps improve fire resilience, enhance infrastructure safety, and protect personnel during fire emergencies. This study provides a valuable scientific foundation for improving fire safety management in these tunnels, with practical implications for designing safer infrastructure and implementing effective fire prevention strategies.