Supercritical water gasification technology is widely used in the conversion of organic waste because of its clean and efficient characteristics. As a high polymer, plastic may undergo complex processes such as depolymerization and gasification in supercritical water, and its optimal operating conditions have rarely been reported. In this paper, the experiments of high impact polystyrene (HIPS) plastic supercritical water gasification were carried out at a reaction temperature of 500-800 degrees C, a reaction time of 1-60min, a feed concentration of 2-10 wt% and a reaction pressure of 22-25 MPa. The effects of different operating conditions on gas, liquid and solid products were studied. It was found that the novel phenomenon that carbon microspheres with uniform specifications on the surface of solid residue. Mechanism analysis results showed the plastic depolymerized to form the oligomer, monomer and its derivatives, which were subsequently cracked and gasified, or polycondensed into a nuclear to form carbon microspheres at a certain critical concentration of nucleation. With the gasification reaction proceeds, carbon microspheres with a smoother surface and a more uniform size are formed with a diameter of about 0.8-1.5 mu m. The experimental results showed that increasing the reaction temperature, time and reducing the feedstock concentration significantly improved the gasification performance of the plastic, but the change of reaction pressure had little effect on the gasification performance. Finally, it was found that under the optimal gasification reaction conditions, the plastic carbon conversion rate reached 94.48 wt%. (C) 2018 Elsevier Ltd. All rights reserved.