The dewatered sludge with TS of 14.62% was diluted to 4 different concentrations of sludge (TS: 6%, 8%, 9%, 12%) and were pretreated at 90°C for thermal hydrolysis. The performance of thermophilic anaerobic digestion was evaluated with the removal efficiencies of SS, VSS and biochemical methane potential (BMP) experiments were also carried out at (55±1)°C to investigate cumulative gas production and biogas production rate. The results indicated that VSS removal efficiency increased with the increase of sludge concentration up to the peak of 26.5% when the sludge concentration was 9%. The VSS removal efficiencies of the sludge with TS of 8%, 9% and 12% were over 20%. The viscosity of the sludge with 9% sludge concentration was 18400 mPa•s, which was much higher than 2900 mPa•s when the sludge concentration was 8%. The result showed that the thermal hydrolysis of sludge with TS of 8% had the best performance. The cumulative gas production of the thermophilic anaerobic digestion with only thermal hydrolysis pretreatment of sludge with TS of 8% was 210.24 mL/g-VS, gas production rate was 0.0074 h^-1 and the removal efficiency of VSS was 22.8%.