With the intake oxygen replaced by water or nitrogen, the difference of heat release rate between them is the premixed combustion phase. The ignition delay and peak heat release rate of oxygen replaced by water is larger than that of oxygen replaced by nitrogen due to the thermal effect of IMWI. In addition, the heat release rate in the diffusion combustion phase between them are consistent, which indicates that the heat release rate in diffusion combustion phase is mainly depended on the in-cylinder oxygen concentration, and the thermal and chemical effect of water have little influence on it.