Abstract: Objective As the core hub of the natural gas supply chain, natural gas pipeline networks take the safe, stable and efficient completion of predetermined gas transmission tasks as their core operation objective. Natural gas pipeline networks are characterized by complex topological structures, and there are usually multiple alternative gas transmission paths and corresponding flow allocation schemes for a given gas transmission task. However, most existing flow allocation methods for natural gas pipeline networks focus on economic optimization as the core objective, with significant deficiencies in considering the guarantee capability of the network to complete gas transmission tasks, namely the system reliability. Methods To address the above research gaps, this paper proposes an optimal flow allocation method for natural gas pipeline networks considering system reliability, which consists of three sequential stages. In the first stage, a flow allocation model with the maximization of pipeline network system reliability as the optimization objective is constructed, under the premise of strictly satisfying the flow conservation, pressure boundary conditions, hydraulic working conditions and operational constraints of the pipeline network. In the second stage, to tackle the challenge of quantitative calculation of pipeline network system reliability, a reliability calculation model for pipe segment units based on pipeline load rate variation is established to realize accurate evaluation of the reliability of pipe segment units under different flow allocation schemes. On this basis, the reliability block diagram (RBD) method is adopted to achieve quantitative characterization of the overall reliability of the pipeline network system. In the third stage, aiming at the solution difficulties caused by the strong nonlinear, non-convex and high-dimensional characteristics of the established model, a staged decoupling solution strategy is proposed. This strategy generates multiple feasible scheduling schemes that meet the daily specified gas transmission tasks, and the optimal flow allocation scheme with the maximum system reliability is finally selected through comparative analysis of the system reliability of all candidate schemes. Results The proposed method is applied and verified in an actual in-service natural gas pipeline network system in China. The numerical results show that the optimal flow allocation scheme obtained by the proposed method reaches a system reliability of 0.998902476, which has a more significant safety margin compared with conventional operation schemes. Meanwhile, the proposed method can realize flow allocation optimization with superior system reliability, while strictly satisfying the gas injection and production tasks, hydraulic constraints and operational boundary conditions of the pipeline network. Conclusion By integrating the system reliability theory into the production, operation and scheduling of natural gas pipeline networks, this study can provide theoretical support and technical methods with both safety orientation and engineering practical value for the decision-making of safe operation of natural gas pipeline networks.