Abstract: The properties of gasification for three kinds of sewage sludges with different moisture contents are studied in an external heated downdraft reactor under a stable air flux of 0.05m³/h and a constant gasification temperature of 800℃. The results show that the contents of CO₂, CH₄ and H₂ in gas, lower heating value (LHV) of gas and aqueous yield increase with increasing moisture content, while content of CO and tar yield decrease. Sludge anaerobic digestion process increases the contents of CO, CH₄, H₂ and C_mH_n in gas and reduces the quality of gas. The anaerobic process in wastewater treatment can improve the quality of gas. The quality of gas produced by gasification of anaerobic digestion sludge from A²/O process is higher than that from activated sludge process. With increasing moisture content in sludge, the difference in the contents of CO, CO₂ and H₂ in the gas produced by both anaerobic digestion sludge gasification is gradually increased, and the difference in the contents of CO₂ and H₂ produced by the gasification of digested and undigested sludges from the same A²/O process is also increased, while the difference in CO content is gradually reduced.