The present invention relates to a lithium secondary battery, more particularly, to a lithium secondary battery in which a low-cost positive electrode active material is applied and a negative electrode active material of lithium metal is formed on the positive electrode side, and to a preparation method thereof. The lithium secondary battery according to the present invention can be produced at a low unit cost of production because it employs a complex combined from relatively inexpensive Co, CoO, Co3O4 and Li2O, instead of LiCoO2 which is a common positive electrode active material, and the lithium secondary battery is also easy to mass produce because of its simple manufacturing process since LiCoO2 is synthesized through an electrochemical reaction due to operating of the battery without a separate heat treatment process.