Understanding the micro-mechanism of lignin gasification in supercritical water is meaningful for improving the energy conversion efficiency of biomass. In this work, the molecular model of guaiacyl dimer lignin with g-O-4 linkages is built and the gasification processes of it in supercritical water at 9 different temperatures between 2000 K and 6000 K are studied by ReaxFF molecular dynamics simulations for the first time. The cleavage mechanism of g-O-4 lignin and the generation pathways of gases were analyzed. During the gasification process of g-O-4 lignin, H-2 and CO are abundantly generated, while supercritical water contributes the most H and O molecules for them. Temperature are found to play important role in the products and rate of the cleavage of lignin. (C) 2020 Elsevier Ltd. All rights reserved.