A facile approach for extracting cellulose nanocrystals (CNCs) was presented through hydrochloric acid hydrolysis of cellulose raw materials under hydrothermal conditions. The influences of preparation parameters, such as reaction time, reaction temperature, and acid-to-cellulose raw material ratio, and different neutralization methods on the yield, microstructure and properties were studied. A high yield of up to 93.7%, crystallinity of 88.6%, and a maximum degradation temperature (T_max) of 363.9^oCcan be achieved by combining hydrochloric acid hydrolysis under hydrothermal conditions and neutralization with ammonia, compared with only 30.2%, 84.3% and 253.2^oC for sulfuric acid hydrolysis, respectively. More importantly, good stability of aqueous CNC suspensions can also be obtained due to the existence of ammonium groups, which can easily be removed through simple heat treatment before using the CNCs.