ZnSe@ZnS QDs were synthesized by a simple one-step method. The prepared ZnSe@ZnS quantum dots (ZnSe@ZnS QDs) composite showed a strong cathode ECL emission using potassium persulfate (K2S2O8) as a co-reactant. The poly-(diallyldimethylammonium chloride) (PDDA) reduced graphene oxide (PDDA-rGO) composite provided a larger activity site could conjugate more ZnSe@ZnS QDs and second antibody (Ab(2)) to improve the sensitivity of the ECL immunosensor. The electrode modified with Au NPs was employed for immobilization of primary antibodies (Ab(1)) and enhancing the electronic transmission rate of the immunosensor. A novel ECL immunosensor was constructed using Au NPs as a sensing platform and PDDA-rGO/ZnSe@ZnS QDs composite as signal probe for highly-sensitive carcinoembryonic antigen (CEA) detection in human serum. Under optimal conditions, a range of 0.0001-100 ng/mL and the detection limit of 0.029 pg/mL for CEA were obtained. This ECL immunosensor possesses long-term stability, good reproducibility and high sensitivity. These are promising qualities for detecting biomarkers, drugs, and environmental pollutants.