At solid retention time (SRT) of 20 days, biogas production, volatile solid (VS) degradation and system stability in co-digestion systems of dewatered sludge (DS) and kitchen garbage (KG) were investigated in semi-continuous completely mixed reactors numbered R1-R5 (the DS/KG of their feeding substrate based on wet mass was 1:0, 4:1, 3:2, 2:3 and 0:1, respectively). The results showed that, with larger proportion of KG in feeding substrate, higher methane yield and biogas yield were obtained with lower methane content. For certain reactor at given SRT, KG addition could significantly improve the organic loading rate (OLR) and volume biogas production. System with more KG addition favored higher hydraulic constant k and VS reduction. The hydraulic constant k was 0.25 d(-1), 0.61 d(-1), 1.09 d(-1) and 1.56 d(-1), and the VS reduction was 37.4%, 50.6%, 60.7% and 68.2% for R1-R4, respectively, indicating higher hydrolysis rates with more KG addition, which led to increased VS reductions. With larger KG proportion in feeding substrate, pH, total alkalinity (TA), total ammonia nitrogen (TAN) and free ammonia nitrogen (FAN) showed decreasing trend. As KG addition increased by 60%, pH, TA, TAN and FAN decreased by 6%, 16%, 22% and 75%, respectively. FAN and Na+ respectively were potential inhibitory chemicals that threatened the stability of the mono-system of DS and KG. In comparison with the mono-system of DS or KG, the co-system showed higher stability by diluting toxic chemicals like ammonia or Na+ to much lower levels.