Abstract: The simulation analysis on the effects of O/C ratio and coal concentration in the coal-water-slurry (CWS) on the flow and temperature fields of gasifier, gas composition and carbon conversion in the CWS gasification was conducted. The questions and resolving methods which are often met with the simulation of CWS gasification process at a high temperature and a high pressure with complex chemical reactions were discussed. The temperature, velocity and concentration fields in the gasifier as well as coal gas composition were predicated. The predicated results were very close to the engineering data. Furthermore, the effects of O/C molar ratio and coal concentration in CWS on the CWS gasification process and coal gas composition were analyzed and the measures to increase the effective coal gas components (CO+H2) were proposed.