Abstract: The effect of hydrothermal reaction temperature, time and solid-liquid ratio on the physicochemical properties of excessive sludge was investigated during the hydrothermal pretreatment of excessive sludge. The results showed that the dissolution rate of COD_Cr and the reduction of sludge increased with the increasing of hydrothermal reaction temperature. At 200 ℃, the dissolution rate of COD_Cr was the highest, 96.5% higher than that of the raw sludge. The maximum reduction of sludge was 80.1% and the ammonia nitrogen changed irregularly. The hydrothermal time had little effect on the reduction of sludge and the dissolution of ammonia nitrogen. When the hydrothermal time was 1 hour, the dissolution rate of COD_Cr reached the highest, 96.5% higher than that of the raw sludge. When the solid-liquid ratio was 10%, the dissolution of ammonia nitrogen reached the least of 1.093mg/g after hydrothermal pretreatment. By comprehensively investigating the effect of the three factors on the physicochemical properties of excessive sludge, the optimal hydrothermal conditions were identified at solid-liquid ratio of 10% at 200 ℃ after 1hour of hydrothermal treatment. Under these conditions, the best sludge pretreatment effect could be obtained, and the maximum COD_Cr dissolution ratio thus reached.