Abstract: The content and composition of hot water-soluble organic matters are important parameters for characterizing the bioavailability of organic matters in sludge. The pattern of hot water-soluble organic matters released from the waste activated sludge was investigated, and the effects of different water quenching time (1-24h) and sludge concentration (0.7-36.0g/L) on the release of proteins and carbohydrates were simultaneously evaluated. The released amounts of protein and carbohydrate reached their maximum values of 34.6% and 36.1% respectively, when the waste activated sludge was treated by water quenching at 60^oC for 8hours. However, with the further increase of the water quenching time, proteins were reduced into ammonia nitrogen, and the degradation of carbohydrate started as well. This led to reduction of the water-soluble protein and carbohydrate concentration. Considering the time efficiency of the test analysis and dissolution rate stability, the optimal water quenching time of the hot water-soluble organic matter was recommended to be set to 5h. When treated with different sludge concentrations, lower initial sludge concentrations led to higher dissolution rate of hot water-soluble organic matters. When the initial sludge concentration was 0.7g/L and water quenching for 5h at 60^oC, the percentages of protein and carbohydrate released arrived at 52.9% and 36.6% respectively, which was the maximum value from 0.7-36.0g/L.