The invention relates to a cuprous oxide-based PIN-junction solar battery of a nano structure and a preparation method thereof. The solar battery comprises a substrate, a P-type cuprous-oxide nano-wire array, an insulation layer, an N-type layer, an N-type ohm electrode and a P-type ohm electrode, wherein the P-type cuprous-oxide nano-wire array is grown on the substrate; the insulation layer is deposited on the surface of the P-type cuprous-oxide nano-wire array; the N-type layer is filled outside the insulation layer to form a film layer; the N-type ohm electrode is arranged on the N-type layer; and the P-type ohm electrode is arranged on the P-type cuprous-oxide nano-wire array layer. Due to the adoption of the nano-wire array structure, the junction area of the battery can be improved, the dispersion distance of a carrier can be reduced, the width of a use-up layer can be effectively increased through the PIN structure, and the separation and collection efficiency of the carrier can be greatly improved, so the energy conversion efficiency of the solar battery can be improved. Adopted raw materials are abundant, inexpensive and pollution-free. An electrochemical deposition method, a magnetron sputtering method and an electron beam evaporation method are adopted by the preparation method and can be widely applied to the industrial production, and a promising development prospect can be realized.