Disclosed is a device for continuously preparing an inorganic slurry by a hydrothermal method including a precursor liquid or slurry stream containing a precursor for preparing an inorganic substance, a supercritical liquid stream containing high-temperature and high-pressure water, and a reactor into which the precursor liquid or slurry stream and the supercritical liquid stream are injected, and from which an inorganic slurry obtained as a reaction product of hydrothermal reaction between the precursor liquid or slurry stream and the supercritical liquid stream is continuously discharged, wherein an injection direction of the precursor liquid or slurry stream forms an angle of 0 to 60 degrees with respect to a discharge direction of an inorganic slurry stream (inorganic substance stream) containing the inorganic slurry in the reactor.