The municipal solid waste (MSW) gasification process was numerically studied in the work. The effect mechanisms of particle size, temperature and gasification atmosphere on the production of H2 and CO were investigated in detail. The results demonstrated that the total volume fraction of H2 and CO dropped from 51.7% to 49.7% with particle size increasing from 20 < d < 30 mm to 80 < d < 100 mm under steam atmosphere. With the temperature increasing from 600 °C to 1000 °C, the total volume fraction of H2 and CO was raised from 56.1% to 65.8% under steam atmosphere. Five different gasifying agents: 100%CO2, 21%O2/79%N2, 21%O2/79%H2O, 21%CO2/79%H2O, 21%O2/79%CO2 were simulated, and the total volume fraction of H2 and CO was 51.87%, 19.1%, 56.13%, 48.36% and 42.98%, respectively. In the gasification conditions considered in this work, H + H2O⇔OH + H2 (R84) played a key role in the yield of H2, and the yield of CO was significantly affected by H + CO2⇔OH + CO (R99) and H + CH2CO⇔CH3+CO (R81).