Since electron or hole injection is a limiting factor for most electroluminescen t polymers, a matching of the work function of cathodes and anodes and the LUMO ＼HOMO energy level of polymers is one of the important factors for high perform ance polymer light emitting devices. In this paper, we report the effect of the cathodes on the performance of the device based on saturated red emitter, three- component copolymer poly (fluorine-co-4, 7-dithien-2-yl-2, 1,3-benzothiadiazole- carbazole or triphenylamine) copolymers. By thermal deposition in combination wi th spin-coating of electron injection layer, we investigated the influence of va rious cathode meterials with different ionization potentials in order to optimiz e the balance of the electron and hole injection. The external quantum efficien cy reached 1.49% with luminance 466cd/m2，the EL spectrum peak is loc ated at 657nm with the color coordinate: x=0.69, y=0.30 for PFO-5Cz-15DBT.