Abstract: Facing the simulation application requirements of large complex natural gas pipeline networks of China Oil & Gas Pipeline Network Corporation (hereinafter referred to as PipeChina) and the actual need for intelligent transformation of the pipeline industry, a new core simulation engine algorithm, i.e. sectional increment iteration method, was proposed. On this basis, the industrial simulation engine of the natural gas pipeline network with progressiveness, robustness and openness was completed. Then, the offline simulation software was developed based on the overall architecture and the application architecture design, realizing the autonomous control over the core simulation technology of large complex natural gas pipeline networks. Using the developed offline simulation software for natural gas pipeline networks, design verification was completed for a newly built pipeline, with an average relative error of 0.1%. By modeling based on the all-element Se-Ning-Lan Pipeline and comparing with the results of commercial software, the maximum relative error of pressure and flow of stations along the pipeline is less than 1% in the steady-state and transient simulation. Besides, the offline simulation software is capable of simulating the all-element natural gas pipeline networks at 2×10⁴ km level with a large complex topological structure. Generally, the developed offline simulation software of natural gas pipeline networks could meet the simulation needs of the unified national network in the future, providing professional simulation service for the digital strategy and intelligent transformation of PipeChina.