Disclosed is a hydrothermal synthesis device for continuously preparing an inorganic slurry using a hydrothermal method. The hydrothermal synthesis device includes a mixer to mix at least one precursor solution for preparing an inorganic material, injected via at least one supply tube, to prepare an intermediate slurry, a connection tube provided at a side of the mixer, continuously discharging the prepared intermediate slurry to a reactor, and having an inner surface contacting a precursor solution mixture on which abrasive polishing has been performed, and the reactor performing hydrothermal reaction of the intermediate slurry supplied from the connection tube by receiving a liquid stream heated to supercritical or subcritical conditions using a heat exchanger and connected to the connection tube into which the intermediate slurry prepared from the mixer is introduced and to at least one injection tube into which the heated liquid stream is injected.