Biomass gasification was subject to poor quality of gas with low heating value and high tar concentration. Anaerobic digestion (AD) was a biological process resulting in lignin enrichment and structure change of biomass which were beneficial to further gasification. Without going into any economic details, downdraft fixed bed gasification of corn straw (CS) pretreated by AD process was thus proposed and performed in our lab. The properties of CS and the pretreated CS (ADCS) were comparatively analyzed, and the influence of AD pretreatment with different retention time (7 d, 14 d, 21 d, 28 d) on gasification under 600 degrees C-800 degrees C was investigated. The results indicated that by degrading holocellulose, AD pretreatment enhanced the gasification property of CS with enriched lignin and catalytic elements (P/Ca/Fe/Al), and the destroyed structure with increased surface area for reactants transfer. Lower heating value (LHV) and cold gas efficiency (CGE) increased with AD pretreatment retention time and topped at 14 d, and then slightly decreased. Compared with CS, the values of LHV and CGE derived from short pretreatment time (7-14 d) increased by 0.76-0.82 MJ/Nm(3) and 8.7-10.69% at 600 degrees C, 1.21-1.3 MJ/Nm(3) and 11.71-13.18% at 700 degrees C, and 0.54-0.86 MJ/Nm(3) and 5.49-8.06% at 800 degrees C, respectively. Under the optimum pretreatment time 14 d and gasification temperature 800 degrees C, the LHV and CGE were 6.83 MJ/Nm(3) and 73.62%. Tar content of ADCS decreased with pretreatment retention time and gasification temperature. Under 800 degrees C, with extension of pretreatment retention time, the tar content decreased by 30 wt%, 35 wt%, 45 wt% and 56 wt%. The results seemed to be interesting and valuable in promoting biomass gasification.