Abstract: To comprehensively investigate the impact of length-to-diameter ratio on hydrogen explosion within confined slender pipelines, an integrated methodology combining experimental approaches with three-dimensional numerical simulations was implemented. The influence of the equivalence ratio and length-to-diameter ratio on hydrogen explosion overpressure, flame propagation velocity, and flame structure was systematically examined. Based on the findings, an interval prediction theoretical model for the maximum explosion pressure rise rate of hydrogen in narrow pipelines was developed. The results indicate that the number of flame acceleration events is proportional to the length-to-diameter ratio of pipeline, and the flame propagation velocity is highly dependent on the evolution of flame morphology. Specifically, the flame propagation velocity increases when the flame front protrudes outward and decreases when it concaves inward. During the process of hydrogen explosion, pressure waves propagate faster than the flame front. These waves continuously reflect within the pipeline and interact with the flame front, altering the flame structure and resulting in high-frequency oscillations in the pressure curve. Under high length-to-diameter ratios, the explosion overpressure peak generated at both ends of the pipeline is greater than that generated in the middle. Both the explosion overpressure peak and pressure oscillation amplitude of the explosion exhibit a decreasing trend as the length-to-diameter ratio of the pipeline increases. The theoretical value of the explosion overpressure peak is not influenced by the pipeline length-to-diameter ratio and reaches its maximum under stoichiometric conditions. However, there are heat losses during the experiments, and these losses are more significant at higher length-to-diameter ratios, leading to a reduction in the explosion overpressure peak. Furthermore, the experimentally measured maximum pressure rise rate falls within the range predicted by the interval prediction theoretical model. This demonstrates that hydrogen explosion in narrow pipelines are not purely laminar or turbulent processes but rather complex phenomena intermediate between the two.