The present invention discloses a copper indium gallium diselenide (CIGS) thin film solar cell back electrode. The back electrode comprises a substrate, a metal conduction layer, a barrier layer, an Na alloy layer and an external protection conduction layer arranged in order from down to up; the barrier layer is transition metal nitride or nitric oxide; the Na alloy layer is formed by Na and another alloy element, and the Na content in the Na alloy layer is 2-10% molar ratio; the thickness of the Na alloy layer is 20-50 nm; magnetron sputtering is employed to perform deposition of each structure layer in order on the substrate for completing preparation; the Na alloy layer is taken as an Na diffusion source to provide Na required by crystal growth for the CIGS absorption layer, the barrier layer can prevent the Na from diffusion to the direction of the substrate and also prevent the impurities in the substrate from diffusion to the CIGS absorption layer to realize the accurate control of Na, and the barrier layer prevents selenium element in the deposition process from diffusion to the metal conduction layer so as to avoid reaction between the selenium and the metal conduction layer and ensure the stability of the metal conduction layer in the growth process of the CIGS absorption layer.