Provided are an apparatus and a method for reaction for use in a co-precipitation reaction for preparing a catalyst or a cathode active material for a lithium secondary battery, which injects a raw material (a solution) at least between impellers according to the solution level in a vessel, thereby making a stirring speed uniform and, in particular, minimizing a concentration difference between solutions. The apparatus for the reaction may comprise: a reaction vessel; a stirring means provided inside the reaction vessel and having multistage impellers; and a raw material injecting means, comprising at least one injection nozzle connected to the reaction vessel, for injecting a raw material at least between impellers.