A tandem organic light emitting diode and a preparation method thereof are provided. The tandem organic light emitting diode includes: at least two light-emitting units (11, 12); a charge generation layer (21) disposed between the light emitting units (11, 12); wherein, the charge generation layer (21) includes a mixed conductive layer (211), and the mixed conductive layer (211) is made by mixing at least one material having a conductivity greater than 103 S/cm with a content of 5˜95 wt % and at least one material having a conductivity less than 10−6 S/cm with a content of 95˜5 wt %. The tandem organic light emitting diode is applicable in a display device.