In order to determine the effects of mixture concentration and fuel blending on the combustion performance of a dimethyl ether/methanol fueled HCCI engine, the combustion process under different excess air coefficients and dimethyl ether addition ratios was simulated. The temperature, pressure, pressure rise rate and the heat release rate were analyzed, as well as the fuel consumption path. The results show that the peak values of the pressure, temperature, heat release rate and pressure rise rate decrease with the increase of the excess air coefficient, and the phases are delayed. With an excessive large excess air coefficient, the further oxidation reaction of CO would be hindered and more CO residue would be produce. With the increase of DME addition ratio, the peak values of the pressure and the temperature in the cylinder increase, and the phases of those advance, while the peak values of pressure rise rate and heat release rate decrease; There are three peaks in the combustion heat release rate curve of dimethyl ether HCCI combustion. The first peak appears at 30゜CA BTDC, with the temperature of 804 K, which corresponds to the low-temperature-oxidation heat release of dimethyl ether. The second peak appears at 15゜CA BTDC, with the temperature of 1193 K, which corresponds to the heat release of reactions formatting CO by formaldehyde and other intermediates. And the third peak is the heat release of CO oxidation, when CO2 is generated. The second and the third exothermic rate peaks are the high-temperature oxidation exothermic stage of dimethyl ether. When mixed with methanol, the low-temperature oxidation reaction of dimethyl ether promotes the combustion of the mixture.