A device for preparing a lithium composite transition metal oxide for lithium secondary batteries by using supercritical water or subcritical water, the device comprising: first and second mixers continuously arranged in a direction in which a fluid proceeds, wherein the first mixer has a closed structure comprising: a hollow fixed cylinder; a rotating cylinder having the same axis as that of the hollow fixed cylinder and having an outer diameter that is smaller than an inner diameter of the fixed cylinder; an electric motor to generate power for rotation of the rotating cylinder; a rotation reaction space, as a separation space between the hollow fixed cylinder and the rotating cylinder, in which ring-shaped vortex pairs periodically arranged along a rotating shaft and rotating in opposite directions are formed; first inlets through which raw materials are introduced into the rotation reaction space; and a first outlet to discharge a reaction fluid formed from the rotation reaction space, and wherein the second mixer comprises: a hollow case; second inlets through which reaction fluids produced in the first mixer and supercritical water or subcritical water are introduced into the hollow case; and a second outlet to discharge a lithium composite transition metal oxide prepared in the second mixer. first and second mixers continuously arranged in a direction in which a fluid proceeds, wherein the first mixer has a closed structure comprising: a hollow fixed cylinder; a rotating cylinder having the same axis as that of the hollow fixed cylinder and having an outer diameter that is smaller than an inner diameter of the fixed cylinder; an electric motor to generate power for rotation of the rotating cylinder; a rotation reaction space, as a separation space between the hollow fixed cylinder and the rotating cylinder, in which ring-shaped vortex pairs periodically arranged along a rotating shaft and rotating in opposite directions are formed; first inlets through which raw materials are introduced into the rotation reaction space; and a first outlet to discharge a reaction fluid formed from the rotation reaction space, and a hollow fixed cylinder; a rotating cylinder having the same axis as that of the hollow fixed cylinder and having an outer diameter that is smaller than an inner diameter of the fixed cylinder; an electric motor to generate power for rotation of the rotating cylinder; a rotation reaction space, as a separation space between the hollow fixed cylinder and the rotating cylinder, in which ring-shaped vortex pairs periodically arranged along a rotating shaft and rotating in opposite directions are formed; first inlets through which raw materials are introduced into the rotation reaction space; and a first outlet to discharge a reaction fluid formed from the rotation reaction space, and wherein the second mixer comprises: a hollow case; second inlets through which reaction fluids produced in the first mixer and supercritical water or subcritical water are introduced into the hollow case; and a second outlet to discharge a lithium composite transition metal oxide prepared in the second mixer. a hollow case; second inlets through which reaction fluids produced in the first mixer and supercritical water or subcritical water are introduced into the hollow case; and a second outlet to discharge a lithium composite transition metal oxide prepared in the second mixer.