Supercritical water gasification (SCWG) is considered as an excellent technique with great potential for lignin utilization, and the addition of a Ni catalyst is effective to achieve high gasification yields. To understand the size effect of Ni nanoparticles during the SCWG process, our simulations were performed by reactive molecular dynamics methods, and the detailed pathways of lignin decomposition and hydrogen production were obtained. The cleavage of the beta-O-4' linkages consists of three main pathways, and the size of Ni catalysts influences the cleavage pathways. During the ring-opening process, the Ni catalyst could accelerate the cleavage of C-O bonds and destroy the conjugated pi bond of the aromatic ring. Moreover, the generation of H-2 molecules occurs entirely on the Ni catalyst. The H radicals gradually approach each other via the transformation of adsorption sites, and the diffusion is the rate-limiting step for the H-2 production, especially at the initial re action stage. The results indicate that a smaller catalyst cluster possesses higher activity, and there are more active sites at the Ni surface to weaken C-C, C-O, C-H, and O-H bonds. Through the cyclic use, the stability of the 2.0 nm catalyst cluster is better than that of 3.0 and 4.0 nm clusters due to the lower surface oxidation degree.