Disclosed is a method of preparing inorganic particles using a hydrothermal synthesis device, including introducing a precursor liquid or slurry stream including a reaction precursor for preparation of an inorganic material into a hydrothermal synthesis reactor, introducing a supercritical liquid stream including water into the hydrothermal synthesis reactor, preparing an inorganic slurry by hydrothermal reaction in the hydrothermal synthesis reactor and discharging the inorganic slurry therefrom, and filtering the discharged inorganic slurry, wherein the precursor liquid or slurry stream includes an NH3 source at a high temperature of the supercritical liquid stream and thus clogging of the stream in the hydrothermal synthesis reactor is inhibited by pH changes in the hydrothermal reaction.