Gasification unit is one of the key subsystems in the IGCC power system; the operating parameters of gasifier directly affect syngas quality and performance of whole IGCC system. The system model of gasification unit with coal water slurry gasifier was simulated and calculated using THERMOFLEX software, and the relations of oxygen coal ratio (R_oc), water coal ratio (R_sc), gasification pressure, gasification temperature and cold gas efficiency were mostly researched. The results show that R_oc and R_sc have effect of mutual restriction on gasification temperature, cold gas efficiency and syngas composition. Gasification pressure mainly determines the capacity of the gasifier, little effects on syngas composition.