In this study, a novel solid-state Ru(bpy)32+ electrochemiluminescence (ECL) sandwiched immunosensor for sensitive detection of α-fetoprotein (AFP) was constructed based on poly(ethylenimine) (PEI) functionalized reduced graphene oxide (PEI-rGO) and Au nanoparticles (AuNPs) decorated polyamidoamine (PAMAM) dendrimers. Both PEI and PAMAM are polymers with a lot of amino groups, which are able to serve as good co-reactant to remarkably enhance the ECL signal of Ru(bpy)32+. For improving the poor conductivity of PAMAM, the AuNPs were decorated on the amino groups of PAMAM. Through Au-N bonds, the formed AuNPs-PAMAM was decorated on the PEI-rGO. The obtained AuNPs-PAMAM/PEI-rGO was introduced to immobilize the detection antibody (Ab2). Then, the Ab2 labeled AuNPs-PAMAM/PEI-rGO was modified onto the glass carbon electrode surface via sandwiched immunoreactions. The ECL substrate was prepared by mixing nafion and the complex (Ru-PtNPs) of Pt nanoparticles (PtNPs) and Ru(bpy)32+, which could reduce the consumption of Ru complex, simplify the operation and enhance the ECL efficiency. The experimental results demonstrated that the proposed immunosensor had good response to AFP. The linear range was from 0.01pgmL−1 to 10ngmL−1 with a low detection limit of 3.3fgmL−1. Meanwhile, with satisfying stability, selectivity and reproducibility, the proposed sandwiched immunosensor was presented to possess good potential in clinical detection. Display omitted •PEI-rGO acts as co-reactant and immobilized platform in the same time, which simplifies the preparation of the sensor.•AuNPs-PAMAM can effectively immobilize a large number of the antibody and dramatically enhance the ECL signal.•The immunosensor for AFP detection exhibits high sensitivity, good stability and satisfying selectivity.