Disclosed is a reactor for preparing a precursor of lithium composite transition metal oxide for lithium secondary batteries, the reactor having a closed structure including a stationary hollow cylinder; a rotary cylinder having the same axis as the stationary hollow cylinder and an outer diameter smaller than an inner diameter of the stationary hollow cylinder, an electric motor to generate power, enabling rotation of the rotary cylinder, a rotation reaction area disposed between the stationary hollow cylinder and the rotary cylinder, wherein ring-shaped vortex pairs that are uniformly arranged in a rotation axis direction and rotate in opposite directions are formed in the rotation reaction area, and an inlet through which a reactant fluid is fed into the rotation reaction area and an outlet through which the reactant fluid is discharged from the rotation reaction area.