Objective: As China's urbanization continues to accelerate, cities are expanding in size, populations are becoming increasingly concentrated, and the demand for energy is growing. Consequently, the construction of underground natural gas pipeline networks continues to advance, with steadily expanding coverage and increasing pipeline density. However, safety incidents have become more frequent during the construction and operation of gas pipeline networks, resulting in serious casualties, property damage, and the disruption of urban production and daily life activities. Urban gas pipelines traverse the underground space of cities, making it difficult to avoid intersections with or co-location alongside other municipal pipelines, such as those for water supply and drainage, district heating, and electricity. This is primarily due to limited underground space and a lack of unified planning and coordination among various municipal construction projects, which often leads to mutual interference between different pipeline systems during construction. This complex underground pipeline layout exposes gas pipeline networks to numerous safety risks. Methods: A model for gas accumulation in cable ducts with adjacent cable runs is established using Fluent, a powerful computational fluid dynamics software package with a rich set of physical models that enables high-precision numerical simulations of complex fluid flow and mass transfer processes. Model development considered various factors, including the actual structure of the cable duct, location and intensity of gas leaks, and ventilation conditions within the duct. Through reasonable simplifications and assumptions, a mathematical model was constructed that accurately simulated the real-world conditions. Subsequently, numerical simulations of gas accumulation within the cable duct were performed, and different operating conditions were included to systematically investigate the effects of varying the slope and ventilation rate on the patterns of gas accumulation within the duct. Results: The results indicate that the slope of a cable duct substantially affects the methane diffusion path, accumulation locations, and concentration field distribution. In the absence of a slope, methane is distributed uniformly around the leak point within 100 s. When a slope is present, methane diffusion is deflected by the slope, and the accumulation effect increases significantly as the slope steepens, making it likely for high-concentration accumulation zones to form beneath the supports at the bottom of the cable duct. Mechanical ventilation effectively reduces methane accumulation concentrations within cable ducts and alters the direction of methane diffusion; its suppression effect is influenced by the synergistic interaction between ventilation frequency and duct slope. Increasing the ventilation frequency improves the overall air exchange efficiency of the duct, causing the accumulation zone to shift from the top (under nonventilated conditions) to the bottom near the exhaust outlet. Conclusions: This study provides multidimensional technical support for the prevention and control of gas safety accidents in areas with overlapping utility lines, including real-time gas leak monitoring, intelligent hazard detection, and emergency response technologies, thereby effectively enhancing the precision and efficiency of gas safety management. In addition, it provides a comprehensive and in-depth scientific basis for the safe design of cable ducts, the layout of monitoring points, and the optimization of risk management strategies in engineering applications, thereby facilitating the development of safer and more reliable urban underground pipeline networks to ensure the stable operation of urban infrastructure and reduce risks to people's lives and property.