Based on the threedimensional model of methanol engine, the effects of inlet dissociated gas injection under EGR dilution and air dilution on the combustion characteristics and emission characteristics were calculated and analyzed by CONVERGE simulation software. The results show that the mixture distribution tends to be uniform after methanol dissociated gas is mixed. For the same dilution ratio, the combustion rate of methanol dissociated gas in air dilution increases more than that in EGR dilution. At the same time, CO and HC emissions in EGR dilution are significantly higher than those in air dilution, while NO_x emission is significantly lower than that in air dilution. The reduction of CO and NO_x emissions of methanol dissociated gas blending in air dilution is more obvious than that in EGR dilution, while the HC emission shows the opposite trend. With the increase of methanol dissociated gas mixing ratio, the maximum comprehensive indicated thermal efficiency under EGR dilution gradually approaches that of air dilution, and the difference between them is only 2.6% when the dissociated gas mixing ratio reaches 30%.