To study the chemical reaction mechanism of such a complex system, one needs accurate and efficient tools. In this work, reactive MD simulations on ab initio-based neural network potential energy surfaces are carried out to explore the growth mechanism of PAHs and soot inception. Leveraging the high efficiency of the NNPES, we can perform hundreds of picoseconds MD simulations for large molecular systems that contain more than 4,000 atoms at the resolution of 0.1 femtoseconds. To ensure the statistical significance of the results, we even performed 10 simulations in parallel starting from the same initial model. Starting from small hydrogen carbons and free radicals, we have detected dozens of reaction pathways to the formation of large PAHs that contain up to 9 aromatic rings from the MD trajectories. The results show that HACA, PAC, and CHRCR mechanisms all play a key role in the growth of PAH. Some of the main reaction paths have been confirmed by experiments and quantum chemistry calculations. More importantly, we discovered many new reactions and reaction paths that are critical for the understanding of soot formation. We also found reaction paths that lead to large polynuclear hydrocarbons that are the precursors of soot inception. The contribution of small PAH radicals is particularly important in such reactions.