Supercritical water (SCW) gasification is regarded as the low pollution approach with great potential for biomass utilization. As one of the most important components of biomass, lignin's conversion greatly influences the final energy conversion efficiency of the wood. It is necessary to disclose the detail reactive mechanism of lignin's gasification in SCW. In this paper, studies on the lignin gasification process in SCW are carried out with the reactive force field molecular dynamics method, in which the Nimz model of the lignin molecule is adopted. The detailed chemical reactions and their occurrence frequencies are both obtained. The generation paths of the main products including CO2, H-2, and CH4 are disclosed. The results show that both the yields of all the three main products and the corresponding H-2 proportions in the products increase with the temperature.