A continuous high solid co-digestion experiment of dewatered sludge from an oxidation ditch process and waste fried tofu was conducted at 55°C in the present study. Sludge retention time was 10 days, and the mixing ratio of sludge, dry and raw fried tofu was 1:0.39:0.06 (dry weight basis). Total solids (TS) of substrate was increased gradually (30, 50, 70, 100 g/L) in the sludge digestion, while TS was increased and then decreased in the co-digestion (43.5, 72.5, 101.5, 72.5 g/L). In the sludge digestion, performance was stable when the TS was below 70 g/L (organic loading rate (OLR) was 5.9 kg-volatile solids (VS)/(m³·d)), and methane yields of sludge were 0.05–0.08 L/g-TS. In the co-digestion, biogas production significantly increased by the fried tofu addition. When total TS was increased to 101.5 g/L (OLR 8.8 kg-VS/(m³·d)), accumulation of ammonia and volatile fatty acid was observed, inhibition was indicated. Methane fermentation failed to recover when the TS was decreased to 72.5 g/L (OLR 6.3 kg-VS/(m³·d)). Results of the denaturing gradient gel electrophoresis analysis of the microbial community revealed a significant difference in the bacterial community with the fried tofu addition, while no major difference in the archaeal community was recognized.