Hydrothermal carbonization (HTC) has emerged as a promising way to improve the fuel quality of low-grade energy resource such as sewage sludge (SS) with substantial merits. Thus, it is important to establish the correlations between HTC treatment and subsequent pyrolysis/gasification of SS-derived hydrochars prepared at various HTC temperatures and HTC durations. The results showed that HTC treatment upgraded the fuel quality of SS and led to the evolution of aromatic structures. Such upgrading made SS-derived hydrochars toward the region of lignite in the Van Krevelen diagram. Thermogravimetric analysis indicated that both pyrolysis and gasification reactivity had a generally negative correlation with HTC temperature. Regarding the role of HTC duration, the opposite trend was observed between pyrolysis and gasification reactivities. In general, not only the fuel properties but also the pyrolysis and gasification reactivity were dependent on HTC temperature more significantly than on HTC duration. These findings can provide an insight into thermochemical utilization of SS-derived hydrochars.