The invention discloses a double-layer mechanical grate type garbage gasification and incineration system. The system comprises a double-layer garbage gasification and incineration furnace, wherein the double-layer garbage gasification and incineration furnace comprises a feeding bin, a gasification furnace and an ash combustion furnace which is located right below the gasification furnace; the feeding direction of the ash combustion furnace is opposite to that of the gasification furnace; a stacking sealing section is arranged between the feeding bin and the gasification furnace; a residue pusher is arranged between the gasification furnace and the ash combustion furnace; exhaust gas outlets are formed in the upper end of the gasification furnace and the upper end of the ash combustion furnace; the double-layer mechanical grate type garbage gasification and incineration furnace further comprises a circular air supply system. The garbage treatment method comprises steps as follows: step A, furnace ignition and drying are performed; step B, garbage is combustedin a hearth of the gasification furnace; step C, continuous garbage gasification and incineration treatment is realized; step D, the double-layer mechanical grate type garbage gasification and incineration furnace shuts down after ash combustion. With the adoption of the double-layer mechanical grate type garbage gasification and incineration furnace for treating the garbage, the garbage treatment amount is large, the recovery efficiency of heat is relatively high, and pollutant emission can be effectively reduced.